I use a Rivendell Atlantis as a primary commuter bike and a Surly Crosscheck with Shimano Nexus8 internal geared hub as a backup bike. The Crosscheck with internal geared hub was originally intended as primarily a bad weather bike and around town errand bike. The past year I have used it more often, for about 30+ percent of my commute rides.

I think it's important to ride the backup bike fairly often just to make sure everything is in good working order and to keep it from getting stale and neglected. Otherwise, the day you need to put the backup into service due to a problem with a primary bike, the backup might not be ready to roll.