I was just about to open a feature request for this too.
An important use case is the initial login. If you’re moving forward with email based login, it’s likely that your first email to the end user when the account is created will say something like:
“You now have an account, go ahead and log in with your email address and do a password recovery”
… but you can’t say that. You need to say:
“You now have an account, and here is your userid. You don’t really need it, but unfortunately you’ll need to use it to do your password recovery, but once you’ve done that you can just log in with your email address. But, if you ever need to do a password recovery again you’ll need to use your login id”
Yes… I’m being a bit silly, but I think it’s clear that using the email address for both logging in and password recovery is ideal. Mixing both is not so nice.