At the moment, the only benefit that users can see for registering their site is to get rid of the annoying button.

We should instead talk about the most important reason why you'd want (as a site admin) to register your site:

  To sign-up on the Security Advisories mailing list

So I suggest we rephrase this sentence:

  Please register your site to remove this button

to be something like:

  Please register your site to remove this button and <b>receive advance notifications of security updates</b>

Francois