Automatic changes in the backup photos.
When I import photos to Lightroom I make a backup copy to an external hard drive and that's easy. But when I start editing and rating and deleting them, those changes will not automatically apply to my backups. This means that my backup catalog is sort of out of date immediately. So it would be nice to have LR to apply those changes in two or more destinations simultaneously. That would keep my backup catalog up to date all the time.