Thinking about it I have avoided practicing on oblique targets because shooting someone in the back hardly seems like self defense, but I need to think about this some.
Shooting somebody in the back is a lot like shooting in the front in terms of the organs underneath, only if you shoot midline, you are more likely to hit the spine than from the front. Plus, you have the shoulder blades up high.
As for not being defense, that all depends on the situation. If a gunman is facing away from you and pointing a gun at your wife and you think he is about to shoot her, do you not think it would be defensive to shoot him in the back? If an intruder in your home is moving away from you toward your children's bedroom with a gun/knife in hand, do you not feel the need to stop him immediately before he does harm to your children?
Defense is situational.