What would you use instead of an email address to identify a user upon registration? An email address is something most people have and its a workflow people are familiar with.
I think the point is NOT to identify users, but only to not have spam/bots. Unless it’s a paid service, users should not have to identify themselves. You can keep out bots with CAPTCHA.
Users should not have to identify themselves regardless of the service being paid or free. Because stopping spam and bots is possible without requiring the user to provide an email or a phone number.