Soon after the first Iraq torture scandal, I remember a pompous politician declaring (more or less) that "The reason why we don't torture prisoners is so American soldiers don't get tortured." Whatever you think about torture, this argument seems underwhelming. Why should you expect the enemy would reciprocate? After all, cooperation has already broken down. That's why you're at war!
You could appeal to universal human rights, but that will probably fall on deaf ears. Most people are going to listen to the tribalist who declares, "All I care about is saving American/German/Russian/Roman lives!"
There is however a reason for treating prisoners well that should appeal to any rational tribalist. Even if you only care about the lives of the people on your side, there is a good reason to treat prisoners well. Namely:
Treating prisoners well encourages enemy soldiers to surrender, and treating prisoners badly encourages enemy soldiers to fight to the death.
Notice: This argument works even if the other side proverbially "takes no prisoners." It doesn't matter how bad the other side is. If an army wants to make its job as easy as possible, it will establish a transparent reputation for treating prisoners like kings. In fact, it would be wise to harshly punish its own soldiers for mistreating prisoners.
This interestingly came up in over-rated Oscar nominee Letters from Iwo Jima. The Japanese are stubbornly resisting the American assault, but finally two desert and surrender. Before long, the Americans shoot the prisoners. When the deserters' troop finds their bodies, their officer says "Let this be a lesson to you." Why not fight to the death if the Americans are going to kill you anyway?
While it might seem like dramatic license, this scene is quite consistent with one of my favorite books on World War II: John Dower's War Without Mercy. The Pacific war was vicious on both sides. But the Americans could have made their lives a lot easier if they had made it clear to each and every Japanese soldier that surrender pays.