I understand the logic, but logic tends to go out the window when a guy has a great game going in September. So, you either put him in the bullpen, where he won't be building his arm or routine to be a starter, let him start in the majors, where you basically have a glorified bullpen game every 5 days, or you do what they've been doing and play the long game with his development.
With our history of pitcher development, whatever they decide will be the wrong answer...
Despite our lack of awe-inspiring star power in the rotation, our starters have been decent (top 1/3). It's the bullpen that has dragged us down.
Also, PCA is 3rd in MLB in WAR so far... I don't think there is a more fun player to watch in the game.