Career .831 OPS at catcher, .859 at a season's worth of 1st and. 861 at a season's worth of DH. Really, that supports Taro's argument about the offensive slippage more than anything.
By the way, how is carrying 2 full time catchers plus Montero any worse than carrying 2 full time catchers plus, say, David Ortiz? If you don't add Prince, the team almost certainly wouldn't add a full time DH. Olivo is good enough against lefties to use as a pinch hitter, and keeping Jaso would give the opportunity to rest weary catchers in extras or double headers. Tell me how much more helpful it will be to carry your second back up infielder will be.

I think playing catcher leads to more wrist and hand injuries which adversely effect performance.  Just look at Roy Campanella, who sandwiched a OPS+ season of 74 between two MVP seasons in the 150s or Johnny Bench who sandwiched a pedestrian, for him, OPS+ of 107 between two MVP seasons.  While far from conclusive (catchers aren't the only position with down years due to injury), I think it is indicative of the risks involved with the position.  Heck, Ichiro preferred RF in large part because of the reduced wear and tear compared to CF.  If the position wears on you over the years, why wouldn't it wear on you from April to September?  Some athletes can take the punishment (think Reggie White missing one game over 12 years or Gary Payton playing through years of back spasms), others are fragile (J.D. Drew for instance).  Who knows how Montero will hold up and how dedicated he will be to maintaining his fitness.  All the same, I platoon him at catcher.  
I still think the M's are uncertain about him at catcher, or at least they were uncertain in the past.  If they thought he was a catcher in 2010, there is NO WAY they trade for Smoak instead of Montero.  My guess is he got a lot better and they like his trajectory behind the dish.  I also think the difference in the Yankees and the M's situations are critical in understanding the trade.  Only truly physically gifted catchers debut at 20-22 in the bigs (Mauer and Pudge types).  For most, the bat development is slow enough that they debut at 24-26 (like Ramon Hernandez and Varitek).  The issues with Montero's glove reflect, in part, his gifts with the bat.  The M's can live with the learning curve of a MLB bat and a AAA glove, but the Yankees can't.  This coupled with the age of the Yankees roster and the need to give A-Rod et al. a day off at DH, makes Montero less valuable to the Yankees than to the M's.

Likely not. Too much investment in one defensive position.
Pure C/DH's are no different.  You're better off with the additional guy being a    multi-position glove guy.
But Montero isn't totally a C/DH.  He's a 1B, too.
I suppose you could say that Smoak is playing 140 games at 1B and Montero the other 20-odd. Then you give him 65 behind the plate and 65 at DH.  Voila, 150 games.
But you could be still investing in three catchers that way.
I think he's either principally a catcher, say 100 games, or he's not a catcher at all (other than emergency).
You're going to keep 12 pitchers (nobody keeps 11 very long), plus 9 principal lineup guys.  You keep a 4th OF, a 3B/2B/1B, a SS/2B and a catcher.  If he's principally a catcher, then you can't keep two bench catcher types. If he's principally a DH/1B then you can. 
And it might depend on if there is another M's addition.
Previously I looked at teams like the 70's Reds and their use of J. Bench.  As I think about it, that isn't the best template here.....because they were a pre/non-DH team. Bench HAD to play a position in the field.  Montero doesn't have to, which changes the situation, somewhat.
Are there recent examples of AL teams who (not counting injuy situations) kept 3 guys who caught 40-60 games each? 
moe
 

The 2010 Diamondbacks (Montero: 85 G, Snyder: 65 G before being traded, Hester: 38 G) used such a strategy to work with 2 talented but fragile catchers (Montero missed 57 games due to knee surgery, 2010 was the only year in the last 4 where Snyder did not miss games due to injury).  Snyder batted .231/.352/.426 while Montero batted .266/.332/.438, Hester batted like a backup catcher.  The team floundered, but the injury prone catchers put up solid lines.
The 2006 Padres are the good one.  They started Mike Piazza (.283/.342/.501) at catcher in 99 games (at DH 8 times and as a pinch hitter 19 times), acquired Josh Bard (.333/.404/.537) and started him in 50 games from May on(as a defensive replacement that received at least 1 at-bat an additional 15 times, and as a pinch hitter 27 times), and Rob Bowen (.245/.339/.394) who started only 8 games (but appeared as a sub 86 times, 19 times as a defensive replacement who did not receive an at-bat).  The Padres used 3 catchers to suck one last truly productive year out of Piazza, a career 1/3 of a season out of Bard, and a career back-up put up good numbers while filling in the gaps.  Piazza completed only 29 of his 99 games started at Catcher, Josh Bard started 50 Games, finished 41 and replaced Piazza an additional 21 times, Bowen replaced Piazza or Bard 57 times.That's my vision for the Mariners, the difference is, Jesus Montero, unlike Piazza, will have the DH available all year.
I gave up looking after finding the 2006 Padres, so there may be more, but they illustrate my argument perfectly even without a DH.

Ice,
For me, it isn't about the running game, it's about pitch framing and calling, plus pitcher comfort.
If the pitchers like throwing to Montero, he'll catch.
As it stands right now, playing Montero at Catcher means you put Wells in the lineup, or Liddi/Seager. I like all those guys, have said so.  But IF there's  significant chance that catching Montero reduces his offensive output then I'm WAY happy with a Jaso behind the plate.
moe

I'm curious - we sent Fister to Detroit for a bunch of parts.  If we'd traded them Pineda, maybe we could have pried Avila out of em. 
Let's pretend we could have.  Would you rather have Montero or Avila as a return on Pineda?
Avila just posted a 143 OPS+ as a 24 year old catcher and is a FA in 2016.  His ML batting eye is .55 so far (was .67 in the minors), he struck out a ton, walked a lot, and had impressive power for a catcher.
His ML fielding % is .991, he's throwing out 32% of attempted base-stealers, and gave up 7 PBs and 56 WPs on the year.  He's a bad defender (though improving - thanks Verlander!) and Detroit added Laird back to the fold as a defender to back up Avila (who may get a few more games at DH this year since V-Mart just blew his knee out and is done for the season).
How much worse is Montero?  His CS % is consistently abysmal (~20%), which is definitely worse than Avila.  Enough worse to not let him play the position?  We're gonna have to see.
But Avila's Silver Slugger kinda made up for his "eh" defense, doncha think?  He was a 5.5 WAR player last year.
If Montero gives us 5.5 WAR can you live with his ineptness at throwing out runners?  I mean, it's always possible that he (gasp!) improves his D.
Jaso's a good platoon-mate for Montero because he hits righties well as a LH hitter (which gives him value in the lineup if Montero moves to DH on some of those days).  He's not a great defender though with a CS % about what Montero's is, so he's not exactly the guy you're calling in for a late-innings battery.
So we don't HAVE a great defensive catcher on the roster.  That being the case, our D isn't suffering tremendously with Montero in there instead of Jaso, or Jaso instead of Olivo.
I agree with moe, as long as the pitchers like throwing to him, let him catch, certainly for 80 games or so.
If they don't like throwing to him...tell them to get used to it.  Until he 100% proves he can't be a backstop, he should be trying.
The chance for a bad-fielding, 140 OPS+ backstop is too appealing.  I'd take a 140 OPS+ DH, obviously, but from a catcher?  Hard to pass up, if it is at all feasible.
And if not?  Then he can go the way of Konerko and Delgado and take his thunder to a less-glove-oriented position.  He should still make his mark that way - I just want to see this way tried completely first.
~G

Either catch him 100 games as your principal backstop, or DH/1B him (and he's you emergency catcher).
But don't fiddle around in between.
moe
 

So ONLY a 4 WAR catcher, then.  Yeah, I think the Tigers will live with him coming back to earth.
BTW, top 9 catchers by OPS last year (B-R stats) with 400 PAs:
1) Napoli, 1.046, 5.5 WAR (113 games, 61 at C) 36% CS, 25% career
2) Avila, .895, 5.4 WAR (138 games, 134 at C) 32% CS, 32% career
3) Montero, .820, 4.5 WAR (134 games, all C) 40 % CS, 29% career 
4) McCann, .817, 2.5 WAR (126 games, all C) 22 % CS, 24% career
5) Y. Molina, .814, 3.9 WAR (137 games, all C) 29% CS, 44% career
6) Santana, .808, 3.9 WAR (155 games, 95 at C) 24% CS, 28% career
7) Iannetta, .785, 2.6 WAR (105 games, all C) 30% CS, 25% career
8) W. Ramos, .779, 2.5 WAR (108 games, all C) 32% CS, 25% career
9) Wieters, .778, 4 WAR (136 games, 132 C) 37% CS, 31% career
If you can hit, and catch some, then teams WANT you to catch some.  Not 150 games behind the plate, but some. CS % is not as important if teams aren't gonna run, and few teams run any more.  The Angels do, so that could be annoying, but for the most part...
Call a good game, frame pitches (even though former umps we know say that doesn't matter) and get along with the staff.  Throw out some runners.  And CLUB.  Those are the qualifications for a modern C.  Only if you can't hit do you need stellar defense.  That's how it works at all glove positions, so I'm glad catcher finally caught up.
200-ish 4 WAR catching seasons all time, and we had basically 6 last year.  Since WAR is a counting stat, that should mean we had some guys playing a lot of games last year.  It didn't work out that way.
If you can stop the ball from rolling to the backstop and have some pop in your bat, you can don the tools of ignorance for a hundred games.
And that's what Montero should do for us. That's how the game is played.  Hopefully he gets better as a catcher, but most of these teams are running their offense-first C out there regardless, and hoping he learns by doing it a lot.
Because he'll be doing it a lot for as long as he's not a complete detriment.
~G

From tangotiger is here.
I'm not gonna jury-rig the chart (since I'm not the math whiz around here) to include all outcomes in one number but the value for stealing a base goes down proportionally as outs pile up, so considering all possibilities (1 outs, 2 outs, double steals, first vs second) I believe it still comes out to about .2 runs per steal.  Some other guys around here could give you an exact figure. 
Here's a link to the value of a steal in an Ichiro discussion:
http://www.lookoutlanding.com/2009/2/27/774722/a-little-more-on-ichiro-an
That might help, and I'll use their figure of .19 runs per successful steal.
So 10 free bases = 1.9 runs allowed if a steal is valued at .19 as it is in that article. It takes 10 runs to = 1 WAR.  So it'll cost Montero a fifth of a point of WAR to suck hard at throwing out base-runners.
Gunning down runners looks really cool, and I won't speak to the momentum change it can create in specific games to get a pitcher out of a jam or what-have-you.
It doesn't make a huge difference over your schlub replacement in run value, though, so at the moment it's not something I worry about with Montero.  I would have been fine with V-Mart throwing out 20-25% of attempted steals and I'm okay with Montero doing it.
He'll make up the -0.2 WAR that his arm causes somehow, I have a feeling.
~G

I think Dave Cameron may have covered this briefly at fangraphs.
Boston allowed a major league high, 206 stolen base attempts, while the league average was 151.  The Red Sox allowed 45 additional stolen bases, coming to a total of 8.55 runs by the same valuation you used, still not a huge problem, but much larger than you make it out to be. 
I think the cause for this is in who decides to run.  The elite guys (30+ SB) will find their opportunities to run, and their getting caught will largely be a matter of luck (pitch selection/location, bad jump, timely pitch out).  It's the lesser runners(5-20SB) that will take advantage of a low risk situation to move up.
The second factor I find important is the steal of 3rd that puts a runner in position to score on an out.  Jarrod Saltalamacchia allowed 16 steals of 3rd while catching 2, Olivo (who had his worst year for it last year) allowed 9 out of 10 attempts, and Jaso, who's not supposed to be very good at throwing out baserunners, allowed 5 of 6.  So the guys we have were not dramatically better than the league worst, which we could assume Montero would be (though that is 7 more guys standing on 3rd than Olivo left in similar chances).  This brings me back to the 3 catchers strategy, because late in late close games (that we will still likely have) it would be handy to get even the little advantage that Jaso or Olivo would bring in holding runners.

I agree that having a weak arm behind the plate may cause teams to run more than they usually would, which will exacerbate the SB issue.
But Jaso is not demonstrably better at C than Montero, and I don’t even expect him to make the 25 out of ST.  I think he’ll be optioned to Tacoma (as he should still have an option left) and he’ll wait for his chance later this year or next April to be Montero’s backup.
We’re gonna get run on.  Pitchers are gonna have to hold runners, because the catcher will not be striking fear into base-stealers.  Olivo has a decent arm but isn’t great at throwing out runners either, so the 70ish games I expect him to catch won’t be much better than Montero’s.
It’s just an aspect of the team.  It would be an aspect with Jaso + Olivo, so we were already prepared to live with it.
 Montero being back there doesn’t change it.
 ~G

... to give than to receive...
(Motto of all Bat-first Catchers named Jesus) 
:)

After pitchers, that's who managers would like to pinch hit for, but they are prevented by the everpresent worry of having no one else on the team to back up there. 
It limits the bench, yes, but you still have enough space to back up at all 3 outfield positions (Wells) and all 3 non-1st Base infield positions (Kawasaki or Seager/Figgins).  There's not a lot of depth on the team, but that doesn't matter too much because Wedge doesn't like to sub in even as much as other AL managers.  The Mariners Sub percentage of all plate appearances was 1.99% compared to a AL average of 2.59%. 
If the last bench spot doesn't go to Jaso, who does it go to?  Most likely a 5th outfielder, possibly Robinson or the recently acquired Darren Ford, either of which would be providing the same service in Center as Jaso would be providing at Catcher;  a late inning defensive replacement or extremely occasional pinch runner

will be DH'ing quite a bit.  Jaso's role is safe on the team unless he is terrible during Spring Training.

Jaso is not Gerald Laird, who was brought in to help Detroit's defense with a shaky Avila back there.  Laird consistently throws out 40% of base-stealers, but can't hit his way out of a paper bag.
There's a place for that guy - backuing up an offensive stud who struggles behind the plate a little more.
We don't have that guy.  Why would I need Jaso as a late-innings replacement?  He's not faster than Montero or Olivo.  I don't understand the point of carrying three slow, inept defensive catchers.  You steal ABs from Jaso that he might need to improve both offensively and defensively.  Olivo's a proud guy and he's gonna have trouble getting less than 50% of the catching games, so he might not...but either way Jaso is then stealing games from Olivo or Montero, but not enough to stay in a groove.
If Jaso played great D, I'd get it.  He doesn't.  Yes, he's a lefty which is nice, but Montero hits righties slightly better than lefties, IIRC.  He doesn't need a platoon mate. 
I'd rather Jaso catches 120 games in AAA prepping for his backup role in 2012 than that he catches 30 for the Mariners and is available to pinch-run.
I don't want to pinch-hit for Montero.  If we pinch-hit for Olivo, then Montero can move from DH to catch and we can go through the bench.
I just don't think Jaso provides us with a ton of flexibility for taking up that 3rd spot, I guess. 
~G

How many games do you expect Montero to catch?  I expect 60+ and hopefully at least half the games.  In a perfect world it'd be more like 100 for Montero and 60 for Olivo, but I don't expect that.  Miguel has his veteran feathers to be stroked so he'll probably be the "main" catcher for 90 games and Montero will catch 70.
Are we expecting to pinch hit for Olivo a lot?  If not, what do I need a 3rd catcher on the roster for?  Yes, Montero will be DHing, but he can slide to C if Olivo gets removed from the game.  Having to pinch-hit or let a pitcher bat for a plate appearance or two doesn't seem like the worst problem in the world.  The NL survives it for 162 games, we can live with it for 1.  Jaso's a phone call away in Tacoma if Olivo is too injured to go the next day.
And that's a problem that EVERY team faces (what happens when one of my catchers goes down).  Having Montero DH half the time doesn't exacerbate from that problem.
What am I missing?  What's the difference in effectiveness between having our alternate-backup C in Tacoma vs. on the 25 man?
~G

You do need somebody who is able to strap on the facemask and chest protector as an emergency catcher.....Think Tui,
but not many teams carry three primary catchers.
I don't think we will.

That was a poorly stated argument.  When I suggested that Ford or Robinson would be providing the same service as Jaso to the team, I wasn't suggesting Jaso was a terrific defensive replacement (though Poor is better than Terrible), I was suggesting they would be the same in that a theoretical 5th outfielder would be backing up 1 position.
The 3rd catcher idea isn't just about a better defender, it's about catcher health, we've had several long discussions about shortened careers and season ending injuries being much more common among catchers.  We talked about how Olivo crashed and burned just last year and perhaps the year before in Colorado from overuse.  This is about keeping Montero (and to a much lesser extent; Olivo) swinging as healthy a bat as possible throughout this year and the next 5 at least.  All I want is for Montero to catch the first 7 innings of about 70 games, the whole 9 of another 20 or so and DH the rest.  Olivo and Jaso can start/finish the rest, and are probably decent enough hitters that the occasional DH or PH spot is not out of the question, especially if Franklin recovers to 2010 instead of 2009, and let's not forget our 25th man hitting shortstop(s).  Other than Liddi, there's probably not a better hitter ready in the organization besides Jaso to add anyway.

The DH is less flexible in the majors than it is in college ball, so it is a real issue.  Here's an example that's on Wikipedia:
In the second game of a doubleheader between the Minnesota Twins and Chicago White Sox on July 6, 2007, the Twins initially used their starting catcher, Joe Mauer, as the DH because Mauer had started the first game at catcher. The starting catcher for the second game, Mike Redmond, however, was forced to leave the game in the first inning due to injury after accidentally being struck in the head by the bat of Jim Thome, and Mauer had to take the field as the replacement catcher. Twins starting pitcher Matt Garza thus was forced into the batting order and became the first American League pitcher to bat in a regular-season American League game since Hipolito Pichardo on July 31, 2000. Garza went 0-for-2, but picked up the win in a 12-0 victory over the White Sox.

Twins tried to sneak by without a backup C, and didn't make it.  Hard to see an MLB manager to agree to be in this situation every time Montero is at DH, unless Monty will DH only a handful of times.
If we envision him as splitting between C and DH, then I think there will be third "real" catcher on the roster.
Jaso could be a valuable LH benchie (and played some 1b in the minors), but, problem is, the manager will be reluctant use the 3rd C as a PH or sub because then you're right back in the same situation -- once you move Monty to C you've got to let your pitchers hit.
The more I turn it over, it just seems really difficult to split a guy between C and DH in light of the way the rules are without really hamstringing your bench.
I'm guessing that if they decide he's not going to play as much C as Mauer does (110+/year), then they'll pull the plug entirely.  They won't make the manager play more than 20-25 games on that kind of tightrope.

Gimenez had to bat when he couldn't swing, and finish the game while Olivo was sitting unable to play on the bench due to hamstring issues if I remember right.  Joe Mauer was listed Day-to-Day on 3 different occasions with, basically, bumps and bruises, Matt Wieters twice, Carlos Santana once, Chris Iannetta twice; and those are just the times they are listed unavailable.  The night that Chris Gimenez had to play through a strained oblique, Olivo was technically listed ready to play.  Catcher injuries are not a one-in-a-million chance, they're a constant worry of managers, which is why it is extremely rare to see a catcher replaced in game when there isn't an on-field injury.  The fact that catchers do face constant performance hindering injury is another reason to carry a 3rd catcher and insulate Montero as much as possible.

Well said Beni,
One double-header, 5 years ago, somebody had to bat their pitcher.
Spec, what you're missing is the DOZENS of times that having that extra bench IF/OF glove or speedy legs came in handy for the Twinkies, just in '07 alone.
G's right, the M's are either catching Montero quasi-fulltime or they'er 1B/DH-ing him fulltime.  He's not catching just 40 games a year. 
Personally, I would be just fine with letting Olivo go and catching Montero 100 games with Jaso the other 60.  I'm just as fine (maybe even better) with DH/1B'iig Montero 155 times.
If we do the first, then teach Seager or Liddi how to put on facemask and chest protector.  The'll do in a pinch.
 
moe
 

First off, Mauer in the past has DH'd no more than 20 or so times a year.  Not suggesting that there would be issues with Montero doing that without a "safety net."  But 50 or 80 or 100 would be different.
Second, I'm not saying it "makes sense" to plan for the unusual situation, I'm saying the manager, psychologically, is unlikely to agree to it.
You can prove with statistics that it's better for a football team to never punt.  Show me a coach who's going to agree to a roster with no punter on it.
 

The reason it only happened once in 7 years is because (rightly or wrongly) managers never allow themselves to be put in that situation on a regular basis.  If every team had only 2 catchers and one of them DH'd 50% of the time, then it would happen more often.  And no manager wants to be the one with his job on the line when his pitcher has to hit in a close game.
Another reason it's so rare is because there aren't many C that you absolutely don't want to lose their bat from the lineup.
Obviously, if Mauer can't continue to catch 120 games a year, then the Twins will be in the same boat.  Will they have him DH 50-100 games per year and not have a third catcher?  We'll have to see.  I bet on the third catcher.