The Yankees have not handled Joba very well up to this point. Firstly, his rookie season, he has to immediately convert to a one-inning pitcher after going 9-2 in the Yankees farm system as a starter. He flat out dominated, just one earned run in 24 inninigs pitched, and an absurd 0.75 WHIP. Then after staying in the pen for the first part of '08, he has to convert back to a starter. Changing from a reliever to a starter is harder than it sounds. Joba even had to add a whole new pitch to his arsenal (curveball).
The only weakness he had starting was his pitch count kept running up too high and too soon in the game. Notice: his pitches per inning average hardly changed converting from a reliever to a starter.
As a reliever, you can afford to waste pitches to get a batter out, you cannot do that as much when starting - especially when you aren't used to throwing more than 20 pitched in a day, like Joba.
Let's face it, Joba's got the stuff to fit in either in the pen or in the rotation. He is sporting a 1.53 career era from the pen, and a 2.76 career era as a starter. Now, what is more valuable to a team? An 8th inning pitcher? Or a guy who's gonna give you six to seven strong innings every fifth day?
If the Yanks keep him in the rotation for all of '09, then he will be less prone to injury and will probably be the best #4 starter in all of baseball.