Would you trade Ellsbury and Carlos Guillen for Quentin and Stephen Drew in a keeper league? Guillen is not my starting SS and Drew would not be either. I do use Guillen to plug into a bunch of different positions on off days which I would not be able to do with Drew.
First off, let me make one point clear: your erstwhile trading partner must not realize that Guillen isn't going to be SS-eligible next year. In any league. He doesn't have a single game at the position, and he isn't likely to. Guillen's likely 80-120ish ranking is a heck of a lot more valuable as a middle infielder. Basically, he's going to be a mid to late-round filler next year. So, this trade is basically Ellsbury for Drew and Carlos Quentin. While Quentin is slumping of late, on paper he's not much of a downgrade from Ellsbury at four offensive categories. But how badly do you need Ellsbury's steals? Drew isn't going to steal too many bases, either.
So, this basically comes down to your team needs. If you can fill your stolen base needs elsewhere, I'd do it. If you're not contending this year -- and yes, it's early enough in deep leagues to make that call -- I'd do it. If neither of the above are true, you should keep the best player involved in the deal.