In all fairness, in games that Beckett has started, the Sox are 5-3. In those three losses, in one game he gave up 2 runs, in another he gave up 0 runs, and another he gave up 9 runs.
While you could argue that he weakened the bullpen, he has only been directly responsible for one loss. I'm not saying he's been pitching well-- he hasnt-- but the team has backed him up.
Edit: Hell, now that I think about it, the one game he lost, he pitched extremely well until he got pissed off and started wailing on Yankees--something many Sox fans wouldn't mind.