I think the theory is that it is never worth it to overpay for a closer. At the same time, I imagine the sabermetric analysis assumes that there is SOMEONE in your pen who is better than an average schlub in high leverage situations.
If the Sox had a Bard-esque reliever who was just pitching in the 7th instead of the 9th, perhaps a closer by committee would work. However, they don't have a good setup man or closer and they don't have anyone who is a beast in high leverege situations. I think Morales is the closest they have.