Well.... I own a number of bikes that are very similar. But I don't pretend to believe that I have them each for a certain reason. I could do very easily with any one of them.
I have them for very simple reasons.... I like them.
Generally they are a timeline of my riding history that I don't want to part with. And it's fun pulling put something I rode many years ago and reliving those memories. Maintance,brake downs...blah. Thats a crock unless your lazy, procrastinate or just plain inept with a tool box.
The only time a true 'backup' bike held true for me was back when I was racing competitively weekly and constantly traveling/flying/on the road.