I guess this whole Peavy debate comes down to which philosophy you subscribe to. One philosophy is that the ultimate goal is the World Series and that if you're pitching isn't set up to win the first series then your chances of advancing in the playoffs are greatly diminished. So much so that you'd risk not making the post-season in an effort to set up your playoff rotation.
The other philosophy is that you can't win the World Series if you don't make the playoffs. The goal of the regular season is to get into the playoffs and then anything can happen once you get in. This is the one I believe in. Very little thought should be given to the playoffs until you clinch.
Nate Silver of BaseballProspectus.com does a good job simplifying the arguments:
Here's Nate's prediction for tonight's game: