I voted "no", as I do not see the point in carrying an extra mag(s), if you have a gun that you have run enough to verify it's reliability. Can problems occur with mags, sure, but the odds are probably even more remote than ever having to draw your weapon in the first place. Personally, I prefer to travel as light as possible in all situations, so just carrying a sidearm alone is more than I'm used to. If I ever have to draw it, and the 11 rounds of 9mm aren't enough to handle the situation, then chances are that I was doomed anyway, so I will take my chances on that. I mean, we are all here because we believe in the 2A right to keep and bear arms, but at some point, I think it can be taken a bit far.
I mean, why stop at a second magazine, why not keep a SBR'd AR15 tucked in a shoulder harness under your sport coat, just in case SHTF while you are at the office.