That is the most important thing that Flex brings to Flash: professional grade tooling. The Flex framework, which Adobe has open sourced, makes application development quick and painless for programmers familiar with event driven programming.
One of the things that fascinates me about the Flash Platform is the number of different types of web professionals it attracts. Those web professionals have very different goals so we largely try to stay out of the way an let them create what they want. But to make them productive we create tools and sell them. But trying to have one tool that does everything isn’t practical. Flex was always intended to be a developer-centric way to create Flash applications and I think Flash Builder 4 is going to show that off better than the previous versions.