As has been stated, for the convenience of making unexpected stops at the range then that's understandable but not so much for self defense purposes unless you legitimately believe we will be invaded & our military overrun (I don't see that happening). Whatever is in the gun + an extra mag. I just can't realistically invision a situation where I would empty 2 mags & then have to reload them & go again.
You've obviously never had to repel communist invaders from your home town in the pacific northwest while yelling "Wolverine!". Let me tell you, you'll be happy to have a stash of several thousand 5.56 rounds, and RPG spares when it's your turn. I know from experience. Patrick Swayze and I were up on that ridge for a long time, with the only thing to eat a deer we killed and skinned from when our dad taught us how. Oh wait... trouble separating movie fantasy from reality again...