I think it's important to mention that the quote you linked to from the Geneva convention doesn't rule out torture in general, but rather, just torture in order to get information:

"No physical or mental torture, nor any other form of coercion, may be inflicted on prisoners of war to secure from them information of any kind whatever."

And that's an important thing to note, because if we lump mental torture in with all torture, then we must realize that, in fact, any and all forms of punishment are actually torture. That includes prison time, fines, any form of public shaming, or even just a mean look. All those things will likely have the effect of making someone feel bad, and are thus forms of mental torture.

I think we might indeed do well to ban and reject all forms of physical torture under any circumstances. And we might also be able to ban mental torture for purposes of extracting information - because, as you said, it seems not to be effective.

But if we ban all mental torture, even for purposes of crime deterrence - then I think we probably just haven't thought through the implications of our words well enough.