I agree with you, because I'm not high on WMB, but I can see the argument for why the Sox could be worse. WMB could emerge as a 30 HR .320 OBP player. I think that if he played up to that level he'd be more valuable than Drew. In this scenario, Bogaerts would be primarily at SS, which maximizes his value. Also, the possibility of a regression from Drew is always possible. He had a 6.7 UZR/150 last year, which could regress to average as he gets older. Steamer and Oliver both are projecting a decline in his defense in 2014. They also project him to be much worse offensively, as they forecast over a .60 point decline in slugging. He ultimately is projected to a 1.4 WAR by Oliver, and a 2.0 WAR by Steamer. Middlebrooks is projected to exceed that total, although not by much. I think they are too pessimistic for Drew, but the possibility that Drew is the worst of the three players does exist. He'd also kick Bogaerts to 3B, which would reduce his value.
Personally, I think they should have the 1/10 on the table, and never play him against LHP. He could also be used as trade bait if Middlebrooks shows potential. However, I don't think 1/10 for a player like Drew would get it done. I have to think the Mets, or Blue Jays would make him a better offer.