I think most people want to have sub categories and not subforums.
Maybe it's just a definition problem:
categorie: some kind of heading to sort forums thematically. >ou cannot make a post to a categorie
forum: contains all threads to a specific topic/theme. So you can post in a forum.
I fear that introducing subforums in a forum would confuse most people.
Example:
# Category: Aviation
# Forum: Airliners
# Subforum: Boeing vs Airbus topics
# Subforum: Lockhead vs xxx topics
You could post to Airliners, Boing vs Airbus or Lockhead vs xxx.
Airliners would contain all posts that are made in airliners (maybe even of Boing vs Airbus and Lockhead vs xxx?) and provide links to the two subforums.
So you have at least two places where to start a new thread. A topic about Boing 747 could be placed in "Airliners" or in "Boing vs Airbus". A moderator would probably move the thread to "Boing vs Airbus" if it's made in "Airliners" because it fits better there.
Long story short, i think that subcategories might be useful but subforums would cause confusion, especially for unexperienced users.
