An advantage of building your unit tests as you develop and tying these tests to some application entry point (a mx:Application tag for flex or a movieClip in a AS3 project) is that the eclipse editor uses incremental compilation to show your errors to you. thus developing something that exists in isolation and gets used nowhere won’t give you proper feedback. having unit tests solves this problem nicely since every class you are writing has a test and every test is linked to the main test runner thus you can be garunteed that every peice of code you write is somehow imported and compilled.