McCain says that information gathered through torture is not useful at all, and he has first hand experience.

Even if it does yield intelligence, it's still comes down to "the end justifies the means". Allowing torture on anyone means abandoning one of the principles by wich the USA claimed the higher moral ground. By allowing torture on suspects you do not only set a precedent, you're also opening the door for worse. It's like a stairway, where the first tread downwards is the most difficult. When you're a tread lower, you realise it wasn't that hard- and what you gained from it certainly is welcome. When you realise there's more further down, you take another tread down. Then another one, and another one, till you either reach the very bottom or fall from the stairs and break your neck, but by that time absolutely nobody's going to feel sorry for you anymore. Principles should never be sacrificed like that. Every step downwards, every erosion invites the next one.

Torture is wrong.