I have 30 chances to hit instead of 8 and can reload far far faster. If the person is wearing soft armor it won't matter. Faster follow up shots.
Can you hit them nine times in a row in a milli second?
Doing a quick dogpile search I found that 00 buck shot weighs 54gr and there is 9 of them coming out of one barrel. Also doing a quick search I found lots of .223 ammo comes in 55gr bullets.
IMHO the shotty destroys the AR15 in most HOME defensive situations. Do you really invision a platoon of zombies kicking down your door or is one maybe two crackheads a more logical scenario.
You do have a point with faster follow up shots but I bet I can get 72 54gr pellets out the 12 gauge before you can get 30 55gr out of the AR.
As far as over penetration goes here is my theory.
First off what are the chances that you actually have to use your defensive weapon? Slim to none.
Then what are the chances that you miss? Definately higher than having to use the weapon but still you could say you hit 50 percent of the time in a stressfull home invasion.
Then add to the chance that you miss and your bullets over penetrate the walls and go into another persons area. What are the chances that they, at that exact moment, that your personal space is broken into and you fire a gun at an intruder and miss them, that it goes through the wall with enough force to hit someone in a fashion that it is fatal or cripling to that individual? I feel that the chances are about the same as this scenario.
I win the powerball for 30 million dollars while home on leave in N.Y. I goto pick up my check at lotto H.Q. and while there I learn my wife and two kids have died from a rabid sasquatch attack that invaded Jacksonville A.R. from the Boggy creek in North Little Rock. Upon hearing of my sad tale, Angelina Jolie happens to be in New York City adopting a little black baby from the upper east side. She feels sorry for me and takes me to her penthouse and has sex with me for three weeks straight to ease my pain. End of story.
Both short tales are plausible but unlikely. That is my theory of over penetration.