Except, it's not a "user token", it's an address. It's as much a user token as a postal address or a telephone number is a "user token": not at all. It's a way to contact a person, not a way to identify a person. Just because you can enforce that only one person with a given postal address can create an account with your service, doesn't make it an inherent "user token".

Sure, but what if two humans share an email address, but both of them want to sign up for your game/service/whatever independently? This is a fairly common situation if you are (say) designing a game that might appeal to kids. Or old people.

If you block that, you're going to lose users. Either because they just can't sign up at all, or because there's too much unnecessary friction in requiring someone to go out and sign up for an email address just to register for your service.

As in bloggsfamily@localisp.net.au that both parents and all the younger sprogs share. Still common as anything.

Probably the most common example these days is spouses who don't email regularly.

Once upon a time, you only got one or two email addresses from your email provider (I'm looking at you, CompuServe and AOL), so at that time it was common for everyone in a family to have a single address. Eventually they added explicit multi-user features -- IIRC, AOL went as high as 5 emails per account before my family moved on.