As usual, take this with a grain or three of salt:
For myself, whether I take an extra mag or not is almost completely based on my guts. If I feel like taking an extra, I take it. If not, I go with the one in the gun. Now, the caveat to that is I currently live in a town that hasn't had a violent crime or even a break-in in the last ten years. We have speeding, running stop signs, and occasionally we have vandalism. As in someone egged the house on the corner a few weeks back.
So, if I'm running around my little burb I rarely carry an extra mag.
However, if I head into Phoenix proper, I almost always carry an extra. And after that little incident on the I-10 a few days ago, I decided to carry that extra mag in an actual mag holster, especially if I'm carrying my Shield or one of my 1911s.
That story was really a reality smack to the back of the head for me. I thought about how I'd feel if I'd just fired on a guy (maybe more than once) and how uncomfortable I'd feel if I only had the one mag for my Shield with an unknown number of other perps scuttling around just out of sight. So now when I have to be in/around Phoenix I carry the extra.