For me it boils down to an argument of offensive vs defensive use of a firearm.
Someone using a firearm in an offensive capacity, have a premeditated control and awareness. They choose the time, place and circumstances revolving around the action of violence they are about to initiate. Therefore, will carry as many magazines and as much ammunition as they see fit. Regardless of magazine capacity, they are at an obvious advantage.
Someone using a firearm defensively does not choose the time, place (distance or available cover) or circumstances (number of assailants, what kind of weapons they are using, whether or not they are wearing body armor, etc...) when they must defend their life. They only ammo available to you, may only be what is in your firearm when you grab it. You may not have the time or luxury to secure reloads.