The main downside is that some folks think they need high-cap mags.........
I propose they need marksmanship training - or better friends!
Not really. What if you have to engage multiple attackers. What if it takes 3-5rds to stop an attacker? Well there you can engage maybe 2 attackers before needing a reload. I look at it like this, no one ever died from having too much ammo in their gun.