I won't ever open carry, but, if I did, I would want one on my gun.
And a retention holster.
And more retention training.
YMMV, but I like the idea of being able to drop the mag if the gun gets out of my holster in a snatch attempt and go on immediately to other things (Americana arm bar comes to mind, or some chokes) while giving up possession of the now-inert brick.
But, since I won't open-carry, it's a moot point....
(Edit: I think the odds of me getting into a bad situation because I couldn't shoot for a second or two while reloading AFTER 17 OR 18 ROUNDS ARE ALREADY FIRED is less likely than finding yourself in a snatch situation as an open carrier.)