It's pretty easy to carry a second mag so the question should be why not.
Just that much more weight pulling your pants down. A loaded 17 round magazine is pretty heavy in the pocket. With 18 on board the Glock 17 already, I don't particularly feel the need for it, coming from my last CCW which was a six shot .38. That's three times the firepower without a reload. It's like a Model 10 with two speed loaders, except you don't need to speed load them.
Now, when I carry the Glock 43 instead, I almost always carry a spare mag in my pants pocket, because those, fully loaded, aren't very heavy.
Of course there's always an extra loaded 17 round Glock mag somewhere nearby, such as in the night stand drawer, or glove compartment of my car, which can easily be grabbed and stuck in a pocket if the need arises.
