At the last defensive training class I took, the instructor of course said we should carry an extra magazine. Then he showed a bunch of FBI data justifying close range training. The average gun fight occurs at about 10ft and the defender uses an average of 5 rounds.
So, now my inner mathematician kicks in. For it to even be a gun fight, you gotta shoot 1 round. To get to an average of 5, you got 1+9, 2+8, 3+7, 4+6, and 5+5. I'm thinking a lot of people are just firing 2s and 3s since that seems to be what you most commonly read about in the paper.
So, statistically, even if you shoot more than the average 5, the need for more than the 9, 10 or 11 rounds that you're probably carrying is way small. Multiple that by the probability you'll ever use your gun in the first place and I can be pretty comfortable just carrying the pistol.