My stance on this is that 180 innings is more valuable than 60 - with Papelbon, Ramirez, Saito, and Okajima, the Sox have a very formidable back half of the bullpen with Dan Bard waiting in the wings.
Masterson was a very good SP for the Sox last year. I also think this allows the team to hedge their bets in case Wakefield or Penny have health issues.