...at least until recently, there was a consensus that torture is wrong. Full stop. It was the practice of tyrants and barbarians. But like all moral progress, the consensus on torture is tenuous, and the only way to hold on to it — the only way to expand it — is by insisting absolutely and without exception that we not allow ourselves to backslide. Human nature being what it is — savage, vengeful, and tribal — the temptations are just too great. Small exceptions will inevitably grow into big ones, big ones into routine ones, and the progress of centuries is undone in an eyeblink.
I'd say that's a pretty good explanation of why there can be no exception to a complete ban on torture (except perhaps what the law already contemplates with the affirmative defense of necessity.) As has been demonstrated most clearly over the last several weeks, a "legal" justification for a particular but limited set of torture techniques turns into routine abuse, the application of non-legally justified techniques in non-legally justified ways, and ultimately, the death of terrorist suspects in our custody. Had torture even been employed with the goal of limited application in a completely good faith effort to uncover imminent plots of attack (it wasn't) there's no doubt that the use of torture still would've done what it in fact did do, which was spread from a limited set of high-value detainees held in secret locations to a larger number of a detainees held at Guantanmo Bay, to an untold number of detainees in Iraq and Afghanistan and beyond. Torture is nothing if not pervasive. Where it can be justified once, it can certainly be justified many more times. Especially if it proves to also be politically useful.
Worse yet, as Kevin alludes, the very subject of torture rots the discourse and democratic values of our nation. Where at one time our nation confidently tried and executed war criminals for acts of torture, now we find ourselves in furious debates over what the very word means, rationalizations regarding the motives for conducting it, and debates about its "effectiveness." All this, for the expediency of torturing "high value" terrorists for information that we might also glean from techniques mandated by centuries of evolution in the law and our moral understanding.
Kevin is right. Torture is and always should be illegal; no exceptions. Nothing less is acceptable in a nation that claims to be beholden to notions of democracy, equality, law and accountability.