I'm not sure I believe the common wisdom either. It sounds more like a rationalization to justify not torturing prisoners: a laudable sentiment, but not necessarily effective. I agree that prisoners may say anything to avoid pain, but they are not likely to lie if they know their statements will be subject to verifcation.

Torturers also do not necessarily rely on a "truth or pain" calculus, either, because they know it is not necessarily productive. The application of torture is usually a long, deliberate attempt to break down the victim psychologically so he wants to help his torturer, so he will not only yield secrets, but give his captors insight into the psychology and actions of his side. Very often the torturer knows his prisoner has no immediate, useful knowledge, and his prisoner knows that he knows. It is this feeling of helplessness that eventually breaks even the bravest men, not the pain.