Dan Drezner explains why it is important to debate the efficacy of torture in order to reinforce the taboo against it:

The point is, these are the people who need to be persuaded that even in extreme circumstances, torture is useless because it doesn’t work at extracting useful information. It is through developing a public consensus on this issue that a norm starts to take effect — and, hopefully, policy practitioners internalize that belief.

As Drezner says, that consensus doesn’t exist right now, and it has to be created by proving to as many people as possible that torture is both useless and wrong. For many people, it will be enough to describe the torture and to make plain what an inexcusable violation of human dignity it is. Many people won’t need to know that torture doesn’t “work” to know that it should never be used under any circumstances. However, in order to make sure that torture can’t be brought back under a later administration or in the wake of a future terrorist attack, it is vital to show that torture apologists are simply lying when they say that these methods can yield or have yielded valuable intelligence. Torture can’t do this, and it hasn’t, and it never will.

Torture is absolutely wrong and absolutely useless, and demonstrating the truth of both statements will make clear how completely bankrupt its defenders’ arguments really are. Proving that torture achieves nothing except the cruel degradation of human beings takes away the only argument its defenders have left. It would obviously be better if no one were willing to offer a defense for something as abhorrent as torture, but we know very well that quite a few people are prepared to do that so long as they can dress up what they’re defending in euphemisms and false claims about its efficacy. The point of insisting on torture’s uselessness is to strip away the remaining falsehoods that its defenders use to conceal the ugly reality of what they are defending.