It takes seconds to change a mag and if you had to change a mag three or four times to get 30 rounds off, that could lead to a number of times that a defender could get the upper hand while the intruder was incapable of responding. Sure, it's not much. But let me ask you this. If you were the teacher and had the ability to respond, who you prefer to face the guy with 30 continuous rounds or someone who had to take the time to reach into his coat, grab another mag (three different times) and reload. My guess is this. If that defender was defending your child, you'd prefer the latter.