From Radley Balko:

I should have opposed torture for the same reason I oppose just about every other surrender of power to the government that naive people (in this case, like me) tend to think looks good on paper: Because the government won’t use it competently, because the government will abuse it, and because the government will find new, inappropriate contexts in which to use it.

It’s one thing to argue that torture may be justifiable and effective in a few, limited circumstances. It’s another to believe that once you’ve given it the power, government will only use torture in those same limited situations where it’s justifiable and effective.