Bill James posts two very good articles on relief pitchers. The first looks at how teams came to see relievers as specialists, not just failed starters or starter/relievers. Teams, for a long time, failed to realize a good reliever could help win the pennant. Here James writes about the 1950s:

The point I am trying to sell you is, you had to beat these guys over the head with a tire iron to get them to understand the significance of a good reliever.  It had been 30 years since Firpo Marberry; it had been almost 20 years since Joe McCarthy divided his staff into starters and relievers; it had been seven years since Joe Page emerged, it had been four years since Jim Konstanty was the NL MVP, and they still didn’t get it.   They still thought of relieving as a transitional stage into or out of the starting rotation.  If a pitcher pitched well out of the bullpen, like Ike Delock or Don Mossi or Sandy Consuegra, they would move him into the starting rotation, see if he could handle a “real” job.  Hoyt Wilhelm was moved into the starting rotation in 1959.  Turk Farrell, one of the best relief pitchers in baseball in 1957, 1958 and 1960, moved into the starting rotation for several years.  When he lost effectiveness, he moved back into the bullpen.  Many times they were GOOD starting pitchers.  Hank Aguirre, after good years in the bullpen in 1960-61, moved into the starting rotation in 1962, and led the American League in ERA. 

BillJamesOnline.com

In the second article, he uses the point system he developed to rank relievers to come up with a list of the greatest bullpen aces. One other thing that comes from the data is that from the 1960s through the first decade of the 2000s, better management of relief pitchers led to more repeating from year to year as the top relievers in the game. Managers haven’t prevented injuries to starters, but they get a lot more seasons out of top relievers.