And they generally are returned with a hero's welcome, and some have been brought into leadership positions with Al Qaeda. It's not like the corporate world where being out of the game for ten years makes a difference. They maintain the same jihadist beliefs, and those don't change. The Quran and Hadith from ten years ago is the same Quran and Hadith today.

The reasons for keeping Guantanomo open are better than for closing (such as keeping their influence away from other prisoners, safety of the American public, etc). All prisons have costs associated to them, so those who complain about costs are throwing out a red herring. The prisoners there are actually treated well, so despite being enemy combatants who don't follow the Geneva Convention, they get far better treatment than they would anywhere else in the world.