Here's the way I think of it: by simply carrying a firearm throughout daily activities, you are defying the odds. Because the odds of needing to use the gun in the first place are slim (very slim). Ergo, why not take those odds a step further and defy the slim odds of being attacked by multiple assailants or being ambushed? If I am willing to prepare for the highly unlikely chance of needing to shoot someone (I am), then I will also prepare for the possibility of having to use several magazines.
Of course, one could argue that my logic can be stretched further. Why not defy the near null odds of the Russians invading with tanks and planes in the middle of the work day? Why not have a few RPGs on hand in the office?
Point being, you need to draw the line somewhere. I draw it at several magazines filled with top-line ammo. Some draw it at one magazine, and I respect that. It's all about what you're comfortable with.