Back in the day - in the days of 3 and 4 man rotations, nearly every pitcher that came up started in the bullpen.
Nowadays, teams don't have that luxury. Teams need 6-10 starters to get through a 162 game season. And once you use a guy in the bullpen for a month, you need to send him down, or do something to stretch him back out and get him ready to throw 80-100 pitches.
