Moving past the specific technologies, though, what are the advantages of abandoning our traditional thin-client web model and going back to client-server? One of the most interesting will be the ability to do information aggregation while preserving privacy. Imagine, for example, that I’d like to see a single, consolidated view of all my finances — my stocks, bonds, bank accounts, retirement savings, and credit cards. Using a thin-client approach, I have to give some web site, somewhere, the ability to access all of my private financial information for me; using a client-server approach, my browser itself could go out and retrieve the information separately from each institution and then aggregate it right on my screen. I have all the advantages of a single view, without giving up any personal information.
Privacy is going to be a bigger and bigger deal on the web over the next decade: as technology gets even better at violating it, governments will come under pressure to pass more and more legislation, limiting what corporations are allowed to ask for and do. AJAX in particular, and the client-server model in general, gives us one way to respect privacy without giving up the advantages of information aggregation.
The RESTafarians should be happy as well, since this involves using the browser as an XML+REST client.