I likewise have never heard of anyone carrying more than one key. To me, it seems more like some Batman-like crazy prepared person would do.
Then again, in college I forgot my keys more often than I’d like to admit. I just don’t see how an extra would help. If I forget one, I’ll forget the other. The only reason I’d even remember my key is because it was on a lanyard with my wallet.
I lost the second key for my car so we only have the one. To be honest the idea of carrying more than one car key would never have crossed my mind before reading this thread. To lock my key in the car I’d have to lock the car with the internal lock button and then leave the key in the car as I closed the door while holding the door handle up. In other words, very unlikely.
Just so we’re clear: I have two keyrings because I don’t like dangling five pounds of keys from my car’s steering column when I drive. It’s noisy, and it scratches up the steering column. So, since I already have two keyrings, the burden of carrying one extra key on my house-keyring is miniscule. Basically, I can’t think of a good reason not to do it.
I don’t carry an extra set on me. I hide one for the front door. My car is foreign and it is impossible to lock the keys inside. My dog jumps up and flips the front door lock so I need an extra key outside. He is a little bugger!
I also have my ignition between my seats so I could never start the car with a big key ring.