I don't think the Taliban are really terrorists. They're a political faction.
I doubt these guys were that valuable. If we had much dirt on them they would have gone to trial long ago. The fact that they are still kicking their heals in Gitmo indicates to me that they aren't too important. (besides they've been out of the loop for 13 years, they're going to be utterly clueless when they get back to Pakistan)
Not leaving a man behind > negotiating with terrorists.
Congress can't meddle with the Commander in Chief being Commander in Chief.
*although this does lend some ammo to the argument that Obama could have closed Gitmo long ago if he wanted to*