Moral authority is a fragile trust. Once it is destroyed, it is gone for a long time, perhaps longer than most of us will live. It is almost certain that the generation coming of age now will no longer be able to point to another country's abuses of power with a morally condemning finger. Because of the actions of Bush and his cadre of unethical officials, any attempt to do so will be met with the derision and ridicule reserved for hypocrites.
Yes, the legal side of the argument is that without evidence of torture, no one gets held accountable. And the downside of that is that no one gets to prove their innocence either. As a result, America has lost the moral authority to condemn other countries for torture because we have no proof that we did not do the same.
We will forever be viewed as the country that destroyed evidence of its crimes. Most of the world and most Americans know that innocent people do not destroy things. That is the last refuge of treasonous criminals to avoid hanging for their crimes. And that's how history will write, teach and remember it.