Here's a short article on why a person might need a hi-cap mag. Good material if you have hippie-pinko friends or co-workers who might want to argue.
http://www.americanthinker.com/2013/01/why_does_anybody_need_a_30-round_magazine.html
http://www.americanthinker.com/2013/01/why_does_anybody_need_a_30-round_magazine.html