There are some very good reasons for NOT banning these things. Here are a few of them:
1) High-capacity magazines are far more advantageous for law-abiding citizens than for violent predators. A criminal, or a lunatic bent upon mass murder, plans his action in advance; so he can carry as many of the smaller, 10-round magazines as he wants. It's not that much of a burden, if it's just an occasional, or a one-time deal. On the other hand, most people who carry a gun for defensive use only carry one or two spare maagzines, if any -- so the more rounds they have stuffed in those magazines, the better, if their trip to the corner market should suddenly turn into the O.K. Corral.
2) Criminals and madmen can (and will) obtain high-capacity magazines anyway -- which, again, puts defenders at a disadvantage.
3) Imprecise hits (or ineffective ammo) may require multiple shots to stop an attacker.
4) In the stress of a gun fight, people tend to miss a lot. Nationwide, in the U.S., police officers have a hit ratio of around 15-25% overall. (More like 50%, if the officer is alone, and shooting at closer ranges.) I remember seeing a video in which a police officer exchanging gunfire with a suspect at car-length distance emptied a high-capacity pistol without scoring a single hit. The "spray and pray" approach is not the recommended technique. Nevertheless, with these statistics in mind, preventing law-abiding citizens from owning high-capacity magazines for their defensive handguns seems like a pretty niave & ill-considered proposition.
5) In a situation in which you're defending against multiple attackers, high-capacity magazines can be especially important. Defenders in home invasion scenarios often rip through some ammo.
6) If you have to defend against an attacker that's shooting at you from behind the cover of a wall, automobile, tree, etc., having more ammo on hand can be really important, as it's much more difficult to hit someone when you can only see part of their head. This sort of scenario is likely to occur in any gunfight that isn't resolved in the first few seconds, because bad guys naturally take cover when they see that their intended victim has a gun too, and is about to send some bad news their way.
7) Police use high-capacity magazines, and no one is suggesting that they should be handicapped with mags that only hold ten rounds, even though the sort of violent encounters the average police officer is most likely to get involved in is not so different (from a tactical point of view) from those that armed civilians get into when defending their homes, their families, and their own lives.
8) Banning high-capacity magazines would not solve the problem of mass shootings -- it would only give a false sense of having accomplished something, and prevent us from focusing on measures that might actually make us safer.
9) Banning high-capacity magazines is just weak. It’s the response of fearful people who have no understanding of guns or the realities of violent encounters and self-defense. It’s a bad reflection on us, as a society, because it just tells the (gun-savvy) world that we’re fearful and ignorant. We have to be better than that!