I have been until now a fervent disciple of compositing a graphic instance into a class in every case, rather than making my class extend MovieClip...
But lately my devotion has been shaken and now I am seriously considering extending MovieClip when making any class which has a visual component. Whereas before I would always compose in a MovieClip instance, almost never make my class inherit from anything but my own superclass, and implement interfaces for polymorphism, now I think I will extend, although still implement interfaces for the usual other reasons. I also still plan to be a composition devotee in all other ways - I like composition more than inheritance...