This has also become a requirement for us, also to make it easier for users to reply to notifications/start threads without remembering which email address they have to send from as has been discussed before, but mostly to allow users to log-in with whatever service they want, and almost certainly end up logged into their account.
Definitely, I think the most use this feature would get in our instance is from employees who have a 'true' email in the auth system (such as
email@example.com) but also an alias they use everywhere (such as
firstname.lastname@example.org). Making users not have to remember which their primary on Discourse is, and not annoying them about having sent an email from the 'wrong' address, would be very benefitial for us.
The one area which I haven't seen any discussion around is around merging multiple accounts owned by the same user. It would be great to, after a user has verified they own an address they want to add as a secondary email, but before that address is actually added as a secondary email, check if any accounts exist with that email, and if they do, prompt the user asking if they want to change the owner of any posts owned by that user to themselves, and add all post/category notification levels from the other user. If they accept, Discourse would go ahead and make the ownership and notification level changes, delete the other account, and add the secondary email. If they decline, Discourse informs the user it can't add the secondary email.
It would also be useful if this merging function could be easily called from a plugin, since for us the addition of secondary emails is going to be done automatically through our auth system.