Why'd you have to go and complicate everything, Phil?
I've got two reasons why Joba should stay in the rotation:
1. His performance last year.
2. His performance this year.
Once the Yanks flipped the switch from bullpen to starter last season, Joba was dominant. New York won 75% of the games (8-of-12) Chamberlain, while the man himself racked up 74 strikeouts in 65 1/3 innings.
He was easily their best starter during that stretch, notching a little better than 10 Ks per nine innings just under than a 3:1 strikeout-to-walk ratio.
Yes, his numbers in both those categories were better as a reliever, but not by enough to make a demonstrative difference. Here, check his splits if you don't believe me.
Now, on to this year.
He just hasn't pitched very well -- not terribly, to be sure -- but not very well. He's had trouble getting his velocity cranked up to where it was last year: 95 mph or better consistently. Add that to a little bit of a control problem (10 walks in 16 innings) and what do you get? Better contact from the hitters, that's what. Specifically to the tune of a better than .300 batting average against and just under 11 hits allowed per nine innings.
Point is, why would you want that in the bullpen? Especially considering Edwar Ramirez, Jose Veras, Phil Coke et. al. have been hit-or-miss this season. Do you really want another guy in there who's not gonna fool hitters?
I think not.