Just to clear that up: enemy soldiers were not used in human experiments (by Germany or Britain/US, see answer below for information on Japan). They were handled with great care because both sides knew that if they treat the prisoners in a bad way the enemy will do that too with his prisoners. Both sides agreed upon red cross visits in POW camps to prevent such things.
For human experiments criminals and in the 3^rd Reich often inhabitants of concentration camps were used. Scientists used these people because they had no rights and no one really cared about what happened to them (or it was easy to hide that anything happened at all).
From a scientific point of view human experiments are a very good source of information. It's just that today it's not accepted to harm people for science and of course you wouldn't find many people to volunteer for such things anyway. So today scientists use rats and other small animals which results in slightly inferior results but it's just not acceptable from an ethical point of view to use humans. But in the 1930s large parts of the scientific community found the idea of experimenting on humans very good. In the war upcoming ethical questions could easily answered with: "those results will help us win the war/develop new medicine for soldiers/the enemy does this too" etc.
Further information:
Unethical human experimentation in the United States (not directly related to WW2, but a huge article worth reading)