In a gunfight, being able to reload with new magazines is a real advantage, and this is why police officers and licensed civilians regularly carry spare magazines for their pistols. The deputy sheriff who was my concealed-weapons instructor in California emphasized to our class the importance of always having at least four spare magazines on your belt in case you are attacked.
When magazine bans were first proposed, the theory was that they would make police officers safer when attacked by criminals. But even the Clinton administration’s 1999 study of the federal assault-weapons ban (which banned new manufacture of magazines over ten rounds) found no statistically significant differences in murder rates of police officers because of the new law. This justification for the ban on high-capacity magazines has not survived real-world experiment.
How much actual “advantage” does a high-capacity magazine give to a monster who is shooting unarmed people? Practically none. The victims have no idea whether he is about to change magazines and are therefore in no position to flee or engage in a barehand attack (even if one of them has the remarkable coolness of mind to try something that courageously foolhardy). For practical purposes, a mass murderer with ten-round magazines is about as deadly as one with 20-round magazines. I suppose if you were to impose a really low limit, such as two or three rounds, you would start to make a real difference in these horrors, but that brings us to the other side of the equation.