SH
Bird shot has it's place, but not for a lethal human defense round.
Yes you don't have to worry about over penetration, but you do have to worry about under penetration. You want LETHAL force. I would never shoot at someone I didn't want to kill! You want devastating results IMO.
Please look into bird shot for home defense, I think you will be surprised how ineffective it is. Here is a start.

Fred

http://www.theboxotruth.com/docs/bot14_4.htm