Well wayne think about this:

Categoriy: movies
Then for each big movies you create a subforum.
Currently if you do this, people will not want to post in subforums and you'll end up with these ghost subforums nobody cares because there is no post in it.

With this system, not only the webmaster can move threads in child forums, but users can post in child forum because they KNOW their post will also get read in the mother forum. So user will be able to jump to specific forum or read the mother forum to have an overview. Its only a benefit for the users.

I think you should not see this as something confusing. I think its quite a revolution in BBS system like flat view replaced threaded view.
Once user will get used to it, it will simply push ahed the whole BBS concept.