Actually I need this kind of configuration. Forums are very popular in Turkey and they work like categories. E.g. you want to create a forum about content management systems. First you will create two forums PHP and ASP.NET . Then you will create some sub-forums under both PHP and ASP.NET like "PORTALS, FORUMS, BLOGS etc" and these sub-forums will have their own sub-forums named with names of content management systems. People will talk about these content management systems.
This is just an example (actually I want to create a forum like this with MonoX). I wanted to explain why I need this.