Random thoughts:
Real-world use is showing that .223 "overpenetrates" less than 9mm on average.
Without 922(o) and NFA, semi-auto and select-fire versions of the same gun cost practically the same, a few dollars difference. In that case, there's no reason to not have the option.
Full-auto detractors: you don't want the "happy switch" option? fine. Don't.
I do. Others here do. If I'm opening up on a lethal intruder at oh-dark-thirty at home, "overkill" is not in my vocabulary - he's threatening to kill me and mine, so I want any option that will facilitate saving me and mine as fast as possible. And yes, I can be accurate and controlled on full-auto. The happy switch will most likely be on "semi", but if situation warrants it (and you can't guarantee it won't) I want that "full" setting just a thumbstroke away. And remember: a shotgun stuffed with OO buck is launching nine 9mm balls across a spreading area per shell - I'm not likely to fire that many on one trigger pull on full-auto.
And in recalling that some of our Founding Fathers had cannons decorating their home entryways, I'd like a couple o' Ma Deuce similarly situated. And equally functional.