People have written letters claiming that torture works and therefore is justified. There is some serious doubt about whether it works. John McCain, in his article in Newsweek several years ago, said that it doesn’t.
Even if it might sometimes work, this is a blatant example of claiming that the end justifies the means, doing evil that good may come.
Also, if we will not abide by the Geneva Conventions, why should we expect anyone else to abide by them when dealing with our citizens?