I've long said that we shouldn't waste time arguing that torture doesn't work. For one thing, the evidence for those arguments seems empirically shaky, especially since many people employing them insist on arguing that torture basically never works, rather than that it doesn't work very often and therefore has a bad cost-benefit ratio. For another, arguing that something doesn't work isn't necessarily an argument for not doing it--it could just as easily be an argument for improving our technique. And if advances in brain scanning research let us develop a reliable lie detector, as seems possible in the relatively near future, then torture will work very, very well.
If that happens, we're in a nasty spot. Most people who make this argument do not, in fact, care whether torture works. They would still be every bit as much against it if waterboarding worked perfectly. Yet when they argue about whether torture works, they're conceding that torture's effectiveness is relevant to the question of whether or not we should engage in it. That implicitly means that if torture becomes nearly perfectly effective, they should change their minds--otherwise, it's not a relevant criteria. So if we get that lie detector, they have to explain why we still shouldn't use this very valuable interrogation method--or confess that they're basically opportunists who will say anything that might advance the case. This will make it somewhat harder to convince people to listen to their other, better arguments.
Thus I think it is much safer to keep arguments about torture on solid moral ground: we shouldn't torture because it's wrong.
On the other hand, Jim Manzi offers a different empirical argument. It's anecdotal, but appealling:
Let's assume arguendo that torture works in the tactical sense that I believe has been used so far in this debate; that is, that one can gain useful information reliably in at least some subset of situations through torture that could not otherwise be obtained. Further, assume that we don't care about morality per se, only winning: defeating our enemies militarily, and achieving a materially advantaged life for the citizens of the United States. It seems to me that the real question is whether torture works strategically; that is, is the U.S. better able to achieve these objectives by conducting systematic torture as a matter of policy, or by refusing to do this? Given that human society is complex, it's not clear that tactical efficacy implies strategic efficacy.
When you ask the question this way, one obvious point stands out: we keep beating the torturing nations. The regimes in the modern world that have used systematic torture and directly threatened the survival of the United States - Nazi Germany, WWII-era Japan, and the Soviet Union - have been annihilated, while we are the world's leading nation. The list of other torturing nations governed by regimes that would like to do us serious harm, but lack the capacity for this kind of challenge because they are economically underdeveloped (an interesting observation in itself), are not places that most people reading this blog would ever want to live as a typical resident. They have won no competition worth winning. The classically liberal nations of Western Europe, North America and the Pacific that led the move away from systematic government-sponsored torture are the world's winners.
Now, correlation is not causality. Said differently, we might have done even better in WWII and the Cold War had we also engaged in systematic torture as a matter of policy. Further, one could argue that the world is different now: that because of the nature of our enemies, or because of technological developments or whatever, that torture is now strategically advantageous. But I think the burden of proof is on those who would make these arguments, given that they call for overturning what has been an important element of American identity for so many years and through so many conflicts.