Romero has not walked enough to be seriously considered as an MLB regular. The guy can hit, for sure. But he's AAAA until he learns to walk at least as much as Montero has. Montero earned his callup. Romero hasn't. .343 OBP in AAA will convert into something ugly in the majors. He's got pop, and if he could play CF that would be a place to put him. Or maybe second base. But that job is taken.
Supersub? Maybe. But I think Chris Taylor is better suited for the Willie Bloomquist role. I want a tough at bat from my role players: hit the mistake pitch if it comes, but work the walk if it don't. I'll take Taylor's AAA .377 OBP over Romero's .343 and 28/40 BB/K ratio.
I just don't see a place for Romero on this team. He's a rich man's Alex Liddi. If we're looking for a 5th outfielder, Ruggiano is a better choice.