I've been mulling it over tonight, and the thought occurred to me that keeping Plouffe, Burroughs, and Parmalee over Hughes may be more about boosting the trade value of the guys here.
Think about who we've got moving up through the system. Even if Plouffe can be "ok" in rf, he's not going to stay there long-term with Revere, Benson, Tosoni, Hicks, and others waiting in the wings. Morneau, Willingham, Mauer, and Doumit all could have a future as full-time DH'ers. The Twins have made it clear that they don't see him as an IF starter. At best, he's a backup on the Twins, but other teams could have space for him as a starter. You send him through waivers now, you get nothing. You give him a few months to show some hitting prowess and halfway decent defense, and maybe we can get some value from him in a trade.
If Burroughs plays well, he could platoon with Valencia, but I think it more likely that one of the two get traded. Same deal as with Plouffe, if you send him down now you get nothing, later on he or Valencia could garner something decent.
Just a thought