Any action or inaction in a position of power will always grant you enemies
I think it should give AP bonus of some kind when you are severely hurt.Then it would be in the hands of player to use the APs wisely.
I don't quite understand what your thinking there skycast, I doubt we will see people taking themselves to lower hp on purpose just so they have a little higher DT and DR, they might as well start with full hp and let the encounter/player wear their hp down as they will live longer this way and not your way.
I don't see why it would only be in battle, so unless there is a technical reason (which I would never know about!) then we could get rid of that.