Sully has a post up weighing the pro and con arguments for torture prosecutions, and is firmly on the pro side. He doesn't actually directly address the 'con' argument of "what if the prosecution fails?", though, so I'll do it here:
If a torture prosecution doesn't produce any convictions, and/or torture is legalized in some fashion, the net effect is the same as if no prosecution was attempted at all. The American experiment to prove that humanity can govern himself better than the tyrants can is a failure either way.