It only gets "foggy" when you base your entire opinion on "statistics" only.
David Ross hit .229 as a backup catcher for the Cubs this year, but I'm pretty certain that if you ask anyone in that Cubs organization, they would tell you that he is an invaluable part of that team. Jason Heyward spent one spring training with the guy and decided to pay for Ross to have his own suite on the road for all 82 games.
Dempster had a similar affect on the Sox. You can ignore it all you want. He also actually pitched well for them. I'm not going to get into Peavy's "beyond the stats" value, because I know where it will end up.
Sabermetrics are great, and they have a ton of value in evaluating on field performance, but until MLB starts trotting "robots" out onto the field every day, you can't base a player's value solely on "WAR," as you are clearly doing with a guy like Dempster.