Have you ever migrated a large project from flash professional into a flex development environment?
It's not as simple as you're suggesting, because business-world library elements are often integrated into various base classes and possess ties throughout the project.
If you simply export them into a .swc, they will continue to be associated with those classes, and any code change will require you to re-publish from flash professional.. is flash builder even your development environment if you're not compiling from there?
Perhaps my google-fu is weak, but I've never seen a shred of useful documentation online about this migration process.
The best articles I've come across involve processes such as: step #1 convert all of your library elements into mxml format.
Yes, probably sounds like a lot of fun when you're staring down a couple hundred (or more) of those library elements, each with their own states, layers and base classes. And that's just step 1.