I also figured Joba would benefit from being in the bullpen because it would prevent him from stretching out his innings total too much. Joba by definition is injury prone, and he's dominant enough and he has the stuff to be considered a "franchise player". What I mean is, if the Yankees can hold onto Joba for as long as they can...they will. You'd have slightly different rules for your rent-a-players and your franchise players, and I think that the Yankees need to protect Joba as much as possible.
Seeing as he had arm trouble last year, and his inning total wasn't that high, I think it'd be wise to have him spend some time as a starter and as a reliever to increase his inning total a little bit, rather than throwing him into the water with a huge inning increase. Huge inning increases on a year to year basis are typical red flags for injury, and the Yankees would probably want to avoid an injury to Joba of all players.