Backloaded Contract for Bay?

Obviously there's been a fair amount of speculation about whether the Red Sox are back in the bidding for Jason Bay. Most of the coverage has mentioned that to do so they would either have to move some either pieces or, more likely, backload the contract. But if they backload the contract, they'll create one of these untradeabe contract in the late years when Bay's defensive deficiencies will be a greater issue. I mean, I won't argue if they do, but why would you do that?