email addresses for usernames have the advantage of already being unique. None of this "gregmac already taken, try gregmac23, gregmc_595, or verbingnounXX instead?" nonsense
One argument against emails-as-names is that some people change email addresses frequently - unless you have some way of accounting for that (and you definitely wouldn't use the username as the contact field), the username will eventually go stale for some users.
You should allow changing the e-mail address. Of course, changing it requires the same validation, but I'm not really sure why there'd be an argument against it (though I do note many sites/applications don't have a mechanism for this).
At the same time, as a user, you need to consider your own e-mail setup. You should not use your ISP-provided e-mail address whatsoever, there is absolutely no reason to tie your contact to an account you may not keep.
Likewise, you should not be using your school or work-provided addresses for something that you expect to use beyond going to school or having that job.
If you're changing your e-mail address frequently for personal reasons (eg, someone harassing you?), then you should actually consider creating an account you use only for authentication, and a separate one that is your public-facing persona.
Having separate addresses for your contact and sign-ups is actually not a bad idea anyway. It actually makes it a bit harder for someone to break into your account somewhere (because they don't necessarily know what address you used), and you get the freedom to change/abandon your public e-mail address if necessary.
I mean unique as far as your end is concerned. Short of very invasive technology, there's no way to stop people from sharing a user token if they choose to do so.
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.
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.
