- makes you sound "tough on terrorism"
- wins votes from cowards who crave any semblence of "security"
- might pay off if the ticking time-bomb scenario ever eventuates
- it's immoral
- impractical (leads to bad intel from false confessions, etc.)
- reduces your moral authority and international standing ('soft power')
- reduces co-operation from allies (who wants to share fallible intel that could lead to their own citizens being mistakenly tortured? Just ask Canada.)
- makes more people want to blow you up
Am I missing something? Compare this to a policy of randomly shooting Arabs (perhaps selected from a lottery). This would have comparable benefits -- just imagine a scenario where the would-be nuclear terrorist gets drawn from the lottery! Wouldn't that be so worth it! -- and you don't have to worry about the cost of false confessions. Boy, I could go a long way in the Republican party...
[On a more serious note, go read Obsidian Wings.]