There are many good reasons to adhere to the Geneva Conventions treatment of combatants and prisoners. This one doesn't get mentioned as often as insuring the reciprocal treatment of one's own who are captured, but it's important:
As on the Western Front in the First World War, then, the crucial determinant of an army's willingness to fight on or surrender was soldiers' expectations of how they would be treated if they did lay down their arms. In regard to prisoner killing in the heat of battle, information about enemy conduct was relatively easy to obtain; eyewitness accounts of prisoner killings tended to circulate rapidly and widely among front-line troops, often becoming exaggerated in the telling. By contrast, news of the way prisoners were treated away from the battlefield was slower to spread, depending as it did on testimony from escaped POWs or the letters from POWs to their families relayed by the International Committee of the Red Cross. (It should be borne in mind that both of the latter channels were effectively closed between Germany and the Soviet Union because of the geographical distances between enemy camps and safe territory, and the refusal of the Germans to acknowledge Stalin's belated subscription to the Geneva Convention.) Such information mattered, because treatment of prisoners varied so enormously between theatres and armies, as we have seen. A British prisoner in German hands had a reasonably good chance of surviving the war, as only one in twenty-nine died in captivity; but a Russian prisoner of the Germans was more likely to die than survive. A substantial proportion of the large number of German troops taken prisoner at the end of the war also died in captivity, though the numbers remain controversial.
Niall Ferguson, The War of the World, The Penguin Press, 2006, p. 551.
Ferguson recounts atrocities committed by all of the armies early in the war, and he says that it increased the tenacity of the Axis powers in fighting on after it became pretty clear in 1943 that they could not win the war. He says it wasn't just fanaticism, but a fear of being killed on capture. Toward the end of the war, massive numbers of pamphlets promising humane treatment upon surrender were dropped in both the European and Asian theaters to counter the wide-spread belief—which often has a basis in fact—that capture equaled death. By all accounts, the pamphlets had a favorable effect on Axis soldiers, encouraging many of them to surrender.
Fast forward to today. Do you suppose, boys and girls, that the widespread reports and evidence of abuse of detainees from Iraq and Afghanistan might be affecting the tenacity of the insurgents and the Taliban, giving them extra incentive to fight on and not be taken prisoner? Spot does.
Alberto, and his former sidekicks, professors John "Organ Failure" Yoo and Robert Delahunty, probably should have thought of that! It's just too bad they're such doctrinaire, ahistorical jerks.
Update: Corrected reference to Robert Delahunty, thanks to the sharp-eyed Big E.