The best explanation of "why not" is Tim Dunlop at Road to Surfdom.
Let's not forget that we've had this debate before. Harvard Professor Alan Dershowitz argued in 2003 that Western democracies "should never under any circumstances allow low-level people to administer torture. If torture is going to be administered as a last resort in the ticking-bomb case, to save enormous numbers of lives, it ought to be done openly, with accountability, with approval by the president of the United States or by a Supreme Court justice."
Dershowitz may argue that the process would not lead down a slippery slope, but this is a man who believes Israel to be a prime example of a country upholding human rights.
As Australians, we should listen to the sane voice of Richard Slade of Quakers Hill, letter writer in today's Sydney Morning Herald:
"Clarke and Bagaric need to study history before they say any more about the reasons torture should be allowed. Our species has a long and terrible history of using torture for "noble reasons". History is full of stories of victims who will say anything they think the torturers want to hear in the hope that the pain will stop. Of course, the fanatic is the one person least likely to yield to torture, so what good things will torture achieve? And how does the torturer choose appropriate victims with certainty?"