As Web developers, we have some options these days in the realm of RIA, AJAX being one. So why use Flex? To me, one answer is how Flex separates the presentation and data access layers.
This means that the Flex app is independent of the back-end: You can use any server technology that meets your requirements -- ColdFusion, J2EE, PHP, .NET... Because so much of Flex is designed around XML (even the MXML document), you can use something as basic as an HTTPService call to read and write data to your data store.
Thus, if in a few months your boss comes to you and says, "We're going with PHP," and you've been using .NET, you don't panic. The Flex front-end remains the same -- just change a little of the plumbing to connect to a PHP app instead. That's why the XML data format is so attractive: Any Web app can generate it.
What are some reasons you're using Flex? Feel free to post comments.