I think adding a stud tackle to protect your budding star QB is a good idea, and getting an extra pick for a guy you didn't want anyway is a pretty solid move as well.
Trading down is always the right move especially when rebuilding from the ground up, it starts in the trenches, and I can understand why they wouldn't want an apparent out of shape headcase like Carter. And the NFL has gone from defense oriented to offense oriented so yeah... offensive tackle is a good place to start when you already have your QB. Have a good line and the running back doesn't matter as much. Plenty of good skill players will still be available in later rounds.
Grats to Witherspoon for being a top 5 pick, wow!