God forbid you need more fire power during the long wait for 911 response to your call for assistance.
The problem with that argument is that it undercuts one of our other arguments. That a mass shooter could kill just as many people with 10 rounds mags, swapping guns and mags as he goes. If he can be just as deadly with 10 round mags, why can't you?
IMO... trying to justify mags based on that nasty word "need" is a losing battle. I don't have to prove a need to the government. I'm a citizen, I can buy whatever product I want to buy. If I then use it in a criminal manner, then the gov't can step in.
Instead of "need" on our end, the ultimate SAF question will be, "Under what authority CAN the gov't ban such things?" Factors such as common use, use by the police, prior legal possession of many millions of them nationwide, etc.