I hope this won't come across as impetuous but I have to ask why those who reason that extra ammo is unnecessary. And I can't see how one's choice of caliber would have anything to do with this choice either. Are you not considering the possibility of a failed mag? The possibility of missing? The idea of multiple assailants? Why wouldn't you want have an extra mag or two of your personal flavor as insurance against the unknown?
And I really dislike it when I read of statistics used as reasoning in such conversations. Statistics are for beans and for dead people. I say that because that is usually the only time statistics hold any value. Statistics, they don't get shot, stabbed, raped or slaughtered. Victims do though. And then those victims die and become more statistics.