@julian I realize this is adding to an old post, but the topic is very pertinent to my situation. I want to provide a use case to support the request @grumpi.
Like most here, I'm building my community to accomplish the dual purpose of building a relationship with my customers while allowing them to engage each other. I'm also using it as a platform in which partners (healthcare space) can engage with my community members. The latter point necessitates the ability for a partner organization to create their own group in which posts and discussions are specific to the group. However, persons COULD be in multiple groups. So, basically, I want people to have the user experience of posting "to" a group rather than a category associated with that group. The only way I can think of to do that would be to have subforums instead.