Required Child Elements

Considering how content models usually work, I have to wonder if  
"Required Child Elements" really means "Only Kind of Permitted Child  
Elements".

For example, a listbox has option in its only Required Child Element.
  * Why is an option required? Shouldn't an empty listbox with no  
children be allowed?
  * Are non-option children allowed?
    - If yes, what is the UA supposed to do with the other children?
    - If not, why have 'option' as an explicit role at all? Couldn't  
children of the containers that require options be taken as options  
implicitly?

-- 
Henri Sivonen
hsivonen@iki.fi
http://hsivonen.iki.fi/