I've seen and handled these and I always thought they were great. If you're going to shoot that caliber, ammo is expensive. You'll want to reload. A bag attached to the magwell means you can rip off plenty of shots without losing brass.
Yes, I know there are brass catchers for regular ARs. However, they're bulky, clumsy things that hang off the side. The brass catcher for a 5.7 AR can be much more elegant in implementation.
Indeed, I use a cheap plastic mag with the follower, spring and feed lips removed as a brass catcher.