I think the combining the "no registration at all" with an optional "send me the link via email" is really nice.
Actually of the 5000 people who have "registered", more than half have given their email address... probably means people like optional stuff, I know I do.
Do you then store the email address after having sent the link to the user? Because I don't think people view it as "giving their email address", but more like "send me the link and forget that you ever saw my email address".
Storing emails would be a convenient way to send the link if the user did not send it from the registration page or lost it in the meantime. As a user I would love seeing this kind of identification and would probably set a email account only used to recover links and avoid sharing my main email account.
Actually of the 5000 people who have "registered", more than half have given their email address... probably means people like optional stuff, I know I do.