I'm going to have a back up dress but it's really only a white cotton knee-length 50's style dress. It's not a 'wedding dress' but just a white dress that I've had for years and could use if there was some disaster involving the actual dress.
I've decided that I'll have it ready because of a disaster story which happened to a friend of my FH. She had designed and hand made her wedding dress. But then the night before the wedding their neighbour's house caught fire and some smoke got into their house and left black smoke marks on the dress. Fortunately they were able to salvage the dress by adding some white fabric flowers over the smoke damaged parts, but I can only imagine how stressful that night and morning was for them.
I personally wouldn't spend any significant money on a back up dress but I'm happy knowing that I have at least some kind of white dress that I could walk down the aisle in if anything happened at the last minute to the actual dress.