What BB said. I _like_ my standard-capacity magazines because I don't have to stop & reload so often.
In the famous "Miami shootout", which caused the FBI to make a number of changes in its firearms policies, two agents lost their lives when their revolvers went empty simultaneously. The BG then advanced with his Mini 14 (Mini 30?) and blew them away while they were reloading (and he wasn't).
I'll bet, if they had it to do over again, those Agents would say standard cap magazines are extremely necessary.