Ken Rosenthal is 100% dead-on about the situation. Dusty and Chapman both want him to close for mostly selfish, though different, reasons.
--Chapman doesn't want to put the time in to be a top starter when he's already a top closer.
--Baker doesn't have many years left and wants to win now, which he thinks he can do more quickly with Chapman in the pen.
The thing is, Chapman could very well be leaving a ton of money on the table. He'll top out in the low teens as a closer, when he could potentially be making $20+ as a starter.