You're missing that they don't intend it as an RPC or REST interaction. It's to "stream" updates out of services, that can have multiple listeners.

This is already happening, but in isolated cases. Behind the scenes, Six Apart streams all the blog posts on its services directly to Google Blog search.

There are quite a few well-known services that get more API traffic (search engines, etc, pinging for updates) than actual human traffic. So something like this is ultimately necessary, whether it's XMPP or something similar.

I think they are aware of message queues, didn't they allude to them in the talk? Anyway they billed their talk as two guys who build web services making a tentative foray into the message-queue world, so if you think you can help, send feedback.