Well, if you can't hit your target you would need more chances to get lucky, I suppose.
Does reduced capacity improve one's marksmanship out of dire necessity in some magical, wishful way?
Maybe I'm missing your point.
Honestly if you can not defend yourself with 12 rounds, then you need not to carry in my opinion. Also like I said if you are facing two people within 8-12 feet both having guns, you better be Mr. quick draw or hope they can not shoot worth a damn. This is not the movies, and the average Joe can not take on 5-6 people at the same time. There are so many factors that play into each situation. I'm not saying not to carry an extra mag, or not to buy a gun for the extra capacity. I'm saying that do not choose a gun just based on, wow I can get 20 round mags. Or that gun has a lower capacity, so I do not want it. Capacity is just one factor, among many to consider.