Don't conflate Wave-the-framework with Wave-the-product. A lot of time and effort were spent on Wave-the-framework, whereas Wave-the-product feels like a first cut demo. I assume that Wave-the-product will go through a number of revisions that will be more Docs like, and I expect EtherPad to be a part of that.
Also, Wave-the-product might just be one client Google releases. If they really want to encourage a "Wave ecosystem," they'll likely pump out multiple clients with different focuses (including, hopefully, a new Wave-the-framework-backed Etherpad.)
Again, product versus framework. XMPP as a communications framework has decent adoption, a number of open source servers and clients libraries in different languages. Jabber has limited adoption because its competing in a network-effect field. GoogleTalk is probably the most popular use of XMPP.
But, the meat of Wave isn't XMPP, its the combination of Operational Transforms on a federated XMPP layer with a web interface. Each of these features is equally important, so don't overlook them. OT makes collaboration easy, the web interface is the easiest user experience, and XMPP lets you scale the backend.