Michael, good job bringing some attention to these two new specs!
After having used Webmentions on my site for 2+ years, I think you (and the Trackbacks vs Pingbacks vs Webmentions for WordPress article) are heavily underselling their true value. Yes, in some sense they’re vaguely similar to pingbacks and trackbacks, but Webmentions have evolved them almost to the point that they’re now a different and far more useful beast.
I prefer to think of Webmentions as universal @mentions in a similar way to how Twitter, Facebook, Google+, Instagram, Medium and others have implemented their @mentions. The difference is that they work across website boundaries and prevent siloed conversations. Someone could use, for example, their Drupal site (with Webmentions enabled) and write (and also thereby own) their own comment while still allowing their comment to appear on the target/receiving website.
The nice part is that Webmentions go far beyond simple replies/comments. Webmentions can be used along with simple Microformats2 mark up to send other interactions from one site to another across the web. I can post likes, bookmarks, reads, watches, and even listens to my site and send those intents to the sites that I’m using them for. To a great extent, this allows you to use your own website just as you would any other social media silo (like Facebook or Twitter); the difference is that you’re no longer restrained to work within just one platform!
Another powerful piece that you’re missing is pulling in comments and interactions from some of the social services using Brid.gy. Brid.gy bootstraps the APIs of Facebook, Twitter, Instagram, Google+, and Flicker so that they send webmentions. Thus, I can syndicate a post from my WordPress site to Twitter or Facebook and people commenting in those places will be automatically sending their commentary to my original post. This way I don’t really need to use Facebook natively to interact anymore. The added bonus is that if these social sites get shut down or disappear, I’ve got a copy of the full conversation from other places across the web archived in one central location on my personal site!
To add some additional perspective to the value of Webmentions and what they can enable, imagine for a moment if both Facebook and Twitter supported Webmentions. If this were the case, then one could use their Facebook account to comment on a Tweet and similarly one could use their Twitter account to like a Facebook post or even retweet it! Webmentions literally break down the walls that are separating sites on the web.
For the full value of the W3C Webmention spec within WordPress, in addition to the Webmention plugin, I’d also highly recommend Semantic Linkbacks (to make comments and mentions look better on your WordPress site), Syndication Links, and configure Brid.gy. A lot of the basics are documented on the Indieweb wiki.
If it helps to make the entire story clearer and you’d like to try it out, here’s the link to my original reply to the article on my own site. I’ve syndicated that reply to Twitter and Facebook. Go to one of the syndicated copies and reply to it there within either Twitter/Facebook. Webmentions enable your replies to my Twitter/Facebook copies to come back to my original post as comments! And best of all these comments should look as if they were made directly on my site via the traditional comment box. Incidentally, they’ll also look like they should and absolutely nothing like the atrociousness of the old dinosaurs trackbacks and pingbacks.