ruuuuuuuuuuuuuuuuuuuuuuuuuuuuuuuuuuuuules.

I agree with you completely, leave Joba in the pen.  Having Mo all of these years has proven just how valuable a dominant closer can be.  Joba clearly has the stuff, so why mess with a good thing?
One discrepancy though, Joba actually has 4 pitches.  While he uses mostly his fastball and slider as a reliever he would certainly not be limited to those 2 pitches as a starter.  Guys seeing him for the second time in a game would hardly have seen everything he has to offer because Joba is traditionally considered a starter thanks to having a fastball, slider, changeup and curve.
All of that aside though, I still say forget the gamble.  Go with the sure thing and groom him to replace Mo.
Nice article.