You should design you own MVC "framework" based on your own needs. If you know a bit of design patterns, Flex has a lot to offer natively.
Best thing of designing your own MVC is that it can be a light weighted or complex as you need.
My experience with frameworks is that you basically have to write twice as much code than you would without using a framework. The good thing about frameworks is, that it forces you to work in a consistent way, but if you can work in a consistent way by making use of Design Patterns, best practices and common sense, I would suggest to stay away from frameworks.
