AZ Snakepit looks at the value of a good back of the rotation, especially the pitchers who will act as the sixth starter. They will be the depth called upon in case of injuries or trades. He also makes this point, however, about healthy rotations:

Exceptional rotation stability was certainly one of the things that helped the Giants last season. They became the first National League team since the 1997 Braves to have four pitchers who each made 33 or more starts. Now that, in itself, doesn’t guarantee success. But the last seven major-league franchises who did, all won their division that year: as well as the ’10 Giants, they were the ’08 White Sox, ’04 Twins, ’97, ’93 and ’91 Braves, and ’90 Athletics. And I note, the only time the Diamondbacks have had more than two such starters was in 2007, when Doug Davis and Livan Hernandez joined Webb, and we got to the NL Championship Series.

On a related note, Rob Neyer talks about the money teams can save by keeping their players healthy.