I always carry at least 1 spare magazine or a speedloader/strip if I'm packing a revolver. In the case of an auto, besides the advantage of extra ammo supply many auto malfunctions are caused by the magazine or require a mag drop to clear. In the middle of being shot at you don't have the luxury of stopping to assess your mag condition like you would on the range so the spare is what gets put back in after a malf clearance.