AJAX via XMLAsynchronousMessage

It seems to me that much of the interactions that take place over a relatively synchronousAJAX XMLHttpRequest could just as easily have been modelled as asynchronous messages.

Now, I'll grant that async messages have been hampered by not using a common wire protocol (e.g. AMQP), and that browsers are fundamentally HTTP clients, but I wonder if an event-driven asynchronous model would have made more sense in the long run?