Jonathan Chait in the New Republic:
First, there's no such thing as a government policy of "torturing terrorists. " There's only a policy of torturing people the government thinks are terrorists. Many of the suspected terrorists at Guantanamo Bay, subjected to agonizing stress positions, turned out not to be terrorists--not because the soldiers who captured them were venal, but because they were human.
Second, torture is designed to force prisoners to provide an answer the interrogator already knows. The torturer relents when his subject provides the "correct" answer. Intelligence gathering, by contrast, is designed to garner answers the interrogator does not already know.