I don't, but I don't make fun of people who do. When you have to draw to protect your life, it means that a lot of things are going wrong all at once. If your gun is going to fail for....whatever reason you can imagine, THIS is when it's going to happen. If we carry sidearms in the first place because we live in a cruel unfair world where bad things happen to good people, then it doesn't make any less sense to carry a backup.
The reason I don't carry one is practicality. If you have two guns, this means you have just exponentially expanded the permutations of things that can go wrong. Which gun do I grab, does it have the same manual of arms as my primary gun, should I reload my primary or grab the backup, you have to keep TWO guns secure at all times instead of just one, things like this. When I go to school, I have a bag of books, a laptop, a carry gun spare mags, a flashlight, usually winter clothes, knife, cell phone, Leatherman. Sometimes I'll go straight from school to take the kids shopping. I don't see myself REALLY needing a backup gun with all the other crap I take regularly.