I carry an extra magazine not for the extra rounds but because from my own research the part most likely to fail in any handgun (including Nighthawks, semi-custom, and custom) is the magazine. You'd be hurting in a real bad way should the magazine foul up. I do agree if you need more than 16 you don't need another mag you need a swat team.
I carry a 1911 ccw with 18total rounds (one extra)
and a G17 on duty with 52 rounds..(two extra) my Glock came with 3 mags, the mag pouch holds 2 so I figured why not...