I keep a loaner bike because I'm constantly trying to get my buddies to come ride with me... they usually only last a few weeks, and so my "loaner" is now the go-to bike for anybody who wants to ride with me. It usually goes something like this:
Me: "Come ride with me on Saturday."
Them: "Oh I can't, I don't have a bike"
Me: "I have a spare. Be at my house at 6:00"
Them: "......"