I just made an issue to capture that request, that could definitely be interesting. https://github.com/aaronpk/webmention.io/issues/62

The main reason Webmention is less susceptible to spam than Trackback is, is because with Trackback, there is no requirement that the comment text have its own URL. At least with Webmention, a spammer has to create a URL for the spam comment.

The webmention.io service sends a secret token in the payload so you can verify the request came from there.