When I fired infantry trophy matches with the U.S. Army, we were required to make a magazine change at the 600 yard line or we would never be able to get off our 30 rounds in the time limit. Most of the competitors could accomplish this in less than 3 seconds. Talk to some of the old-timers from World War II who fought with the M1 grand. The M1 used clips and not magazines, but even with that the soldiers became very efficient in swiftly reloading the rifles. This magazine argument holds no water.
I think you concede the point that an inexpert gunman could get off a lot more shots with the magazine than with a clip. The only disadvantage to him would be the possibility of jamming the gun.