Subclassing subclasses of Sprite

I’d just like to double check something with you.

Say you have a MovieClip in your Library in Flash CS3. You want it to subclass a class that you’ve made, like forum.kirupa.Example. Your Example class must subclass MovieClip, right?

But I don’t need the functionality of MovieClip! I’d rather inherit from Sprite. I can have my Library assets subclass Sprite directly, so why can’t I subclass a subclass of Sprite?