For a carbine that's for grabbing and fighting with what's on the gun, and only what's on the gun at super short notice, they're one of a few decent options. Mag couplers/jungle mags, redi-mags, and buttstock pouches are all decent ways to have a reload on a carbine you're going to fight with, without being able to "suit-up" with beforehand.
If you're able to, I'd prefer to keep spare mags on my body. Keep the rifle light and maintain a more streamlined profile if possible.