Two different scenarios.
A person on a shooting spree, especially in a 'gun free zone,' is on the offensive, most likely unopposed, and can take the 2 seconds to reload when necessary. If you watch the available surveillance videos from some mass shootings such as Columbine, you'll see the shooter(s) just casually walking around at times looking for their next victim. They're prepared, they've thought it out, the extra ammo is readily at-hand. So, while there may be an instance when someone might attempt to 'jump' a shooter during a reload, most of the time the 10 (or 7 now in NY) mag limit isn't going to stop them or slow them down (not that they'd be following the law anyway).
On the other hand, a person who has resorted to a firearm for self-defense is (obviously) on the defensive, shittin' bricks, often disoriented from just being woken up, etc. They probably didn't have time to grab an extra magazine, and they're probably pulling the trigger for dear life. The fewer interruptions in their attempt to defend themselves, the more likely they'll survive.
A gun is a tool. All the things that make it a better tool in the hands of person whose use is justified make it worse when in the hands of a criminal. That's the Catch 22. Problem is, when a law gets passed to try to limit the potential for damage, the criminals don't comply. They're stubborn like that.
.
