I got such a great response from my last help request that I'm posting another:
On my journey to try and be a better developer, I'm questioning some of the fundamental ways I work and organise code. Like most Flash developers (I think), I normally extend Sprite or MovieClip when I want to control something on the stage. But would I be better off creating a class that composes (i.e. controls via a reference) a Sprite or MovieClip instead? What are the advantages and disadvantages? Which are you doing? Leave comments please!