Some Flex developers insist an architectural framework (aka framework or microarchitecture) is the fundamental piece of a well-designed and maintainable Flex application architecture. Others insist that frameworks are unnecessary and only contribute to the overall complexity of an application.
Personally, I believe frameworks are a valuable tool and can provide a great starting point for a sound application architecture. If used properly (that's a big IF), frameworks provide structure and consistency, both of which contribute to the predictability and maintainability of a code base. Also, many frameworks provide built-in support for many useful design patterns (i.e. Dependency Injection or Inversion of Control) to help developers solve complex problems.
Regardless of what I think, frameworks exist, and they are being heavily utilized in Flex projects. Let's find out which is currently the most popular, shall we?