I'm not sure this is an either or proposition. I haven't seen many message queue protocols with tools for identity, user authentication, federation, etc. I'm also pretty sure that xmpp doesn't even begin to replace message queuing on the backend, but when you look at what folks like EngineYard are doing with it (basically distributing messages), it seems like the value for them was more along the lines of 1) easy ui integration, 2) well understood problems and tools by developers, 3) well understood servers by admins.
So there we have an interesting mixture of the two technologies, you get the insanely fast message routing and throughput of RabbitMQ while leveraging the tools that xmpp brings with it (users, federation, etc.)
The merging of xmpp and rabbit mq is fascinating -- i've bookmarked this and will follow-up on it. I'm reading through the documentation now, but from what I can see this is great tech. (mod parent up!)
More on the proposal of mixing the two, the rabbitMQ folks have done some interesting work integrating with xmpp. (See their xmpp transport (http://www.lshift.net/blog/2008/07/01/rabbitmq-xmpp-gateway-...), and rabbiter (http://github.com/tonyg/rabbiter/tree/master).
So there we have an interesting mixture of the two technologies, you get the insanely fast message routing and throughput of RabbitMQ while leveraging the tools that xmpp brings with it (users, federation, etc.)