I think that reasoning about morality without an irrational, faith-based approach leads to a "slippery slope" if you will of reasoning. Essentially, I believe that the fundamental utility of religion is that it provides a framework for morality which is external to logic : therefore it allows one to establish moral barriers which cannot be cross regardless of any logical rationalization. This is important when considering things like eugenics - there are rational lines of thought that lead you to very very ugly conclusions. If you have an arational "hard stop" at the personal and societal level, we avoid these situations.