Main bike (off road): does the heavy lifting
Backup (off road): Rides like meh so it encourages me to fix the main bike faster
MUP bike: with friends who no longer hit the trails
Foul weather bike: I need a cycling hit... NOW. and I don't care that everything is going to get trashed/submerged/gritified.
Something different: Use it occasionally, seemed like a good idea at the time.