If torture were effective, it would allow the debate to be framed as a tradeoff. If slicing off the man's left hand would allow you to save X lives with 100% ce

If torture were effective, it would allow the debate to be framed as a tradeoff. If slicing off the man's left hand would allow you to save X lives with 100% certainty, pretty much everyone has a value of X for which they would justify the action -- for most people that value would be quite low. The problem with these tradeoffs is that there's no clear cut moral high ground on which choice is correct -- it's a murky area. So it's a much more compelling argument if you can make the case that the tradeoff doesn't exist at all b/c torture is ineffective.