They seem popular in Canada. Or well, 5/30 there (semiauto rifles are limited to 5 rounds and pistols to 10).
I don’t get it. So long as it reasonably fits out of the mag well, good to go. 10 rounders fit just fine. Anything shorter would probably need to be a blocked 10 round body.
These longer mags give the shooter the benefit of having more to grab when reloading. Flush-fit mags are great for being shooting, but will be a pain if you want to reload quickly for other shooting activities.
So long as they reliably feed, I don’t see the problem with them. I’d rather buy these for competition and training than the flush 10s.
Then again, I also enjoy trips to PA and VA, but that’s just me
IMHO, if it is just for better grip, I’d rather 10/20s. For an AR-15 at least I find 20 round mags to be the perfect size. Doesn’t get in the way on the bench really ever, or prone and still plenty of grip space and decent capacity.