Spammers could erect vast junk sites, fill them with links to valid blogs and spray out webmentions. These webmentions would look valid to most implementations as of today.
DDoS attacks: Using the example above, someone who hates Dr. Drang could send me and a bunch of other bloggers a ton of webmention requests giving his site as the source. All of our blogs would then barrage his site to check the validity of these mentions.