It’s probably a question of how organized you expect users to be in terms of the emails they’re using. Here is a scenario where it would help to have more than one “backup” emails: At my university, every user has at least two email addresses by default first_name.last_name@university.tld and first_name.last_name@department.university.tld. In addition, if you are affiliated to multiple departments or research centers, you might have additional ones. Plus, when a department is renamed the old aliases remain functional.
For whatever reason (I suspect it has to do with how you setup your email an a new device) people are sending emails with different sender email addresses, probably without even noticing it. I have one user who already has two staged users because of that…
I agree that it’s a good idea to start with one backup email, but I just wanted to mention this as an example where more than one makes sense.
Of course, multiple emails won’t entirely solve this either because these people obviously won’t bother inputting all their aliases, but at least it would allow admins to do so to avoid having to deal with the messages from staged users.
Another related scenario is obviously going to exist when it becomes possible to merge users.