There are a lot of reasons that none of my officers use them, other than on backups/off duty, and that's mostly on G27'/26's. For one it's the KISS method, the mags from Glock work correctly 99% of the time, why fix what's not broken.
Weight is a factor, most of us carry the mag in the gun and two on the belt, now they don't make mag pouches, at least to me knowledge, that will fit mags with the extensions on them. So that makes 46rds on us at all times. Since the mag ext would only work for the mag in the gun, is 2 more rds really worth taking a chance on a non factory part?
The other, is that at the range I issue officers "training mags" basically they are older mags with orange floor plates, this is done for two reasons, one the orange plates makes it easier for me to tell who's loaded up and that way our duty mags don't get tore up by getting dropped on the ground, stepped on and such. Also by using older mags it increases the chances of failures, thus helping to reinforce, while training, weapons failure drills. We do however test our duty mags and ammo every so often by shooting up our old duty ammo when it's time for replacement. So I would not want to train with mags that are substantially different than the ones I would be using, as putting ext on training mags would be cost prohibitive.
However for someone who is off duty or CCW and can conceal a Glock well with the ext on it, then yes I don't see why not. Hope that ansers your question.