I used to use only one as well.

I kept a lot of spare parts on hand because I learned that when things break, they have to be fixed in order for me to get to work and shops aren't always open at 9 on a Monday night.

Eventually, my supplies built up so much that all I needed to have a second bike was a frame, and then one became available to me. I suddenly had a back up bike.

I've used my wifes "little old lady" bike a couple of times and even used my sons cheap, department store mountain bike to go to work in the snow.

I recently added a cruiser with internal gearing that I'll use on rainy days, so I guess I have 3 regular bikes and 2 back ups before I have to resort to the car.