Something forgotten by the "Ticking Time Bomb" argument

I've argued before that torture, or "harsh interrogation techniques" (per the Bush-euphemism du jour) doesn't generally work particularly well. But, that isn't to say that it never works- just that on average it is less likely to get reliable intelligence than other methods, or is likely to produce so much bad intelligence as to overtax precious resources that could be used on pursuing more reliable intelligence.

Anyways, the argument of the pro-torture (or "harsh interrogation techniques") crowd often centers on the ticking time-bomb scenario. And, frankly, they're probably wise to do so- in an event where you are talking about an imminent threat of millions of people dying and where you know (or have ample reason to believe) that torture is the only thing that will get you the intelligence you need quickly enough, why wouldn't you torture? Therefore, the argument goes, the President (or appropriate officials down the line) should have discretion to make the call on when torture (or "harsh interrogation techniques") is permitted- without fear of prosecution.

Here's the problem with this argument, though- necessity, self-defense, and defense of others is an affirmative defense already recognized by every single common law court in the US, Britain, and- to my knowledge- the world. Having such an affirmative defense means that, yes, the authorizing official and (maybe) the interrogator, rather than the prosecutor, will bear the burden of proof if they are ever brought up on charges...but isn't that the way it's supposed to work? If you're going to cross the legal line, shouldn't you be required to have sufficient justification for doing so? Think of the affirmative defense as a post-sentence trial (for purposes of this analogy, the torture is the "sentence") of the alleged terrorist, but with a lower standard of proof (either "clear and convincing" or a "preponderance of evidence," rather than "beyond a reasonable doubt").

Why is this already existent way of handling torture issues insufficient to protect national security?