If you could convince people that it's ineffective and counterproductive, they wouldn't even need to be rationalists or even humanists in order to oppose it. So your opposition to torture (which I also oppose btw) doesn't seem like a conclusion that a rationalist is much more likely to arrive at than a non-rationalist -- it seems primarily a question of disputed facts, not misapplied logic.

There's one point that seems to me a failure of rationalism on the part of pro-torture advocates: they seem much more likely to excuse it away in the case of foreigners being tortured than in the case of their own countrymen. If the potential advantages of torture are so big, shouldn't native crimebosses and crooks also be tortured for information? This to me is evidence that racism/tribal hostility is part of the reason that they tolerate the application of torture to people of other nations.

Btw, I find "reduces someone's utility" a very VERY silly way to say "it hurts people".