Here is were I would use it, I have a category "local forums" with 7 or 8 local forums, I would like to add a forum for almost every state, but that would make the main index page a scrolling nightmare, would be nice to just view a category of "Local Forums" then click on sub forums where 50 state forums could be. Yes, I know you can min the category but not everyone can do that (unregistered) or wants to do that.
"Me wants sub-forums"