Dear Mr Cheney
I think you are missing the point. It doesn't actually matter if in some cases, torture resulted in useful information. It's still torture.
Just as, if a person had had the chance to kill Hitler, or Stalin, or Pol Pot, or any of the other people we know were responsibility for the deaths of millions, and had taken advantage of the opportunity, it would still be murder.
We might feel it was justifiable. We might be grateful to that person. We might even consider it just, in a way. But, absent actual trial and conviction, absent a judicial process that we accept as necessary to our way of life, it would still be murder.
It's pretty basic, as far as I can see.