A framework takes care of repetitive tasks. In an RIA, these include building screens, including providing rich controls, validation etc. The framework should also provide an abstraction for maintaining state and requesting data from web services via XHR. While building an OpenSocial application in December, I could have saved a lot of time developing on top of such a framework.
MVC is the dominant model for UI development in the desktop world. A modified form is very popular in web development. It makes a lot of sense to stick to MVC for RIA development. RIAs running in the browser (or Flash or AIR) bear close resemblance to desktop applications.
Topper Bowers just open-sourced his Mamoo framework. Although Mamoo doesn’t seem to be a run-of-the-mill RIA platform, it does provide some of the MVC goodness that those applications need.