In 1988, starting pitchers threw 71 percent of all innings. That dropped to 68 percent in 1998, to 65 percent in 2008, and now it's down to 61 percent. There are two ways to interpret this data.
One way: Starting pitchers as we knew them are dying out. Every year, fewer guys get to 200 innings. Teams are increasingly concerned about pitchers working a third time through the order, and the Rays have made ample use of their "opener" strategy, discarding tradition. Teams aren't creating workhorses and like using relievers, so they accumulate a number of good ones.
The other way: Starting pitchers as we knew them are dying out, but they're not dead. Starters still throw the overwhelming majority of all innings, and a good starter remains more valuable than a good reliever. Various contenders are going to try to upgrade their rotations because having a good rotation is critical -- down the stretch and in the playoffs. Teams can't just lean on relievers all the time.
Rotations still matter, both for teams trying to make the playoffs and for rebuilding teams. Every team still wants to have a good starting five (or six). I've done my best to rank the current starting rotations, but this isn't just a review of how all 30 rotations have already done; it's a look ahead as well. Think of this as a projection as well as an evaluation, based mostly on analysis and a small amount on gut feelings. I am not docking teams likely to make their rotations worse through trades. They haven't made those trades yet, after all.