The problem with less lethal options, be it an under powered gun or rubber ammunition is it brings more uncertainty to the situation.
Now instead of you KNOWING you have deadly ammunition, you are now uncertain if your ammunition will still kill, or will be completely ineffective.
How is your mind going to resolve the uncertainty? Are you going to lower the threshold at which you pull and or use your gun in the hope you can somehow defeat the threat before you are actually in fear for a life? You can see the foolishness of that.
On the other hand, if you pull then gun because there is no alternative, and your attacker is immune to the result, you just escalated a bad situation to a worse situation.
IMHO, either do pepper spray/mace/stun gun/taser, or do a firearm with lethal ammunition. Courts and bad guys recognize the general non-lethal nature of the former, and pulling a gun with out all the faculties needed to use it is a very dangerous decision in my mind.