Russell Branyan and Jose Guillen come to mind as pretty terrific stopgaps that were either at or near the end of their careers when the strolled through Seattle. Having a hard time coming up with others, but that's primarily because I haven't watched but a handful of games in the last seven or eight years.
But in the rotation my memory seems to suport your thesis. Washburn held his own throughout his contract, but his was a four year contract and not a stopgap. We bailed on Gil Meche just before he got into a multi-year boerderline TOR groove. Carlos Silva was a mid-tier disaster but, again, was brought in as a long-term piece ala Washburn. We held onto a bunch of fizzlers like Nageotte, Anderson, Blackley and others when it sounded like significant offers were made for some of them.
It seems like the risk-aversion of the organization has led them to make too few trades, resulting in either a magnification of their unlucky mistakes OR resulting in them making genuinely bad trades when good-for-the-M's trades were up on the board but failed to get consummated for whatever reason. It seems to me that you either have to basically stick to your guns re: prospects, in effect keeping ALL of them, or you need to be far more active than the M's were to get the Law of Averages on your side and spread out your risk.