Wake up Alice! OOA emergencies happen. You can't just say "don't run out of air". It's not suppose to happen, but it does. It happens all the time! It's never happened to me, but it could. There is no excuse to not be prepared for it.
If you are ever in a situation where air is no longer available at depth what are you going to do? You going to just sit there? No, I bet you head for the surface, and if you've had proper CESA training you've got a better chance of making it there alive. It won't guarantee survival, but it will increase the likelihood.
People who are OOA head for the surface. People with CESA training head for the surface safely.
-Ben