My backup bike general consist of 3 groups.

1 The latest purchase I have made to test it on the commute to work comparing it sorta to other bikes along the same route.

2. Testing out a bike I am getting ready to sell to make sure its ready to go. I also deliver bikes to work to their new owners getting a ride home from my wife or on my folding bike.

3. Whatever other bike I have that Is actually my own bike. I might have to commute on my mountain bike next Monday now that I have it all ready to go.

So in short I ride a "backup bike" fairly often. None of them are outfited like my trek with rack basket lights etc and grungy enough that I feel comfortable locking it places. My folding bike gets alot of use as a commuter because I can go to work with my wife in the car and take out the folder when I want to go home early etc.