What common use case is backing up wifi passwords or any passwords, for that matter? Yeah, there are difficulties here, but I don't want or need you to backup my wifi password. Its none of your business.
I've probably saved hours of phone support because my parents wifi passwords and email setups is saved in their iPhone backup (this is only done if the backups are password protected). When they get a new device it's just a matter of plugging it in, choose restore, and that's it. They do have to enter some payment info again for each new device on the App Store though.
