You are right, if the 30 round mag has a problem feeding it is useless and no one gets hurt that day, but the chances of three separate 10 round mags all having problems is very slim.
Reloading a magazine, if done correctly, should only take 4 or 5 seconds to jam 30 more rounds in them. If you go the slow way, reloading a 30 round magazine takes several minutes. No idiot in their right mind would take just a single 10 round magazine and when out of bullets just6 reload the magazine. Anyone intent on creating as much chaos as possible takes along multiple 10 round magazines already pre-loaded so that the amount of time it takes to change out magazines takes less than a split second. You CANNOT bum-rush someone changing out a magazine and expect to be upon him before a new round is chambered and ready to go. The only scenario in which that would be possible is if Bucky were the one with the rifle, he would bumble and stumble giving you ample time to kick him in the groin.
Any moron worth his salt can change a magazine on a AR in a split second, your hand never even has to leave the pistol grip to do it.
Hundreds of videos like this showing how its done.
[video=youtube;ksZqzPWm7VQ]http://www.youtube.com/watch?v=ksZqzPWm7VQ[/video]