I have seen a number of suggestions that, after our capture of Khalid Shaikh Mohammed, that we not be too gentle with him. Or, we turn him over to some of out less scrupulous allies for "interrogation" (wink wink, nudge nudge). Bad idea.
In the movie Dirty Harry, Harry Callahan (played by Clint Eastwood) is chasing a serial killer. The killer has kidnapped a 15 year old girl and buried her somewhere with only enough air for (IIRC) 15 hours. Harry catches the killer and "persuades" him to tell the girl's location (.44 magnum, meet kneecap. Kneecap, meet .44 magnum). Plot ensues.
This is obviously what folks have in mind. KSM may (ha!) have knowledge of al Qaeda plots in progress. We need that information, and there's no time to be squeamish about how we get it. The problem is very simple -- it doesn't work that way. Dirty Harry is a movie, and Harry's getting the information from the killer in about a minute via torture is as realistic as his .44 magnum I've known a number of people who bought "Dirty Harry" model .44 magnum revolvers after seeing the movie. Every one of them has regretted it. They're essentially useless.
Torture is very effective at producing confessions. It is, however, almost totally ineffective at getting correct information. Torture subjects will say anything to get it to stop -- but their information is almost certainly false. You still have to investigate everything, which takes time. You might as well use the resources for real police investigation, and if you're doing that, you don't need the torture. In the case of KSM, we have his notebooks, computer, cellphones, and address books. If he tells us what's going down, cool. If not, we have lots of other people to talk to ...
Basically, torture is useful only for letting some sickos get their rocks off.