I have been using Flex3 for mxml flex RIA development and I do pure AS3 development with Flash CS3 resources, which in most cases I code in Flex Builder but test and compile in Flash AS3. Twisted I know but the reasons are simple:
1. I can't access the fl components packages in Flex Builder easily (I have heard of ways to do so but having had a chance to research).
2. I am often using resources from the library in a .fla file
3. Clients in most cases have Flash but not Flex and expect a FLA
4. I also use systems like SWFAddress so I need to compile to a .swf and test live or test the .swf in a standalone player
So firstly I know flex has its own components that are part of the framework but why not give us Developers easy access to the fl flash components (hence Flash Builder)?
There needs to be an easy way of importing and using a FLA based AS3 project in Flash Builder. I heard about a Flex component that does that but why not at least give access to the library content off a FLA?
I noticed it expects an AS3 project to be set out as Flex Builder does, using an application .as file named as that project. In the launch configuration you need to be able to browse and set your own application .as (that maybe the document class from the FLA). This would help the above request by letting users export/test in their current file/folder structure.
Just my intial feedback.
Also, the best (and most official) way to suggest features is to log an enhancement request in the public Flex bug base at http://bugs.adobe.com/flex/.
You may want to search to see if these have already been suggested and if so, vote on the issue. If not, file a new enhancement request and encourage people to vote on it.