This is a good and interesting topic. Pitchers do tend to find their role in the staff 'rotation', so to speak......and some of them are eventually found to be 'fill ins', throwing garbage innings rather than being put in tight situations.
Position players who are limited to one position may get stuck behind a better player, as Fungo states. Those with some versatility often have advantages that good freshmen pitchers enjoy......the chance to play in a variety if situations as a way to earn full-time playing time.