Just to throw my own perspective in on this subject, I would very much like to have joinable groups in a future release of the software. I have two use cases for this feature, both of which I've been personally involved in overseeing:
1) Distributed convention planning. I'm the chair of a (very) small convention that involves the talents and resources of people from across the country. We use a phpBB forum (because it's free) for our everyday planning processes, and each "department" of the convention planning staff has its own section of the forum, as well as specific access permissions for other sections (to keep people on-task and in their own space). These departments are based on phpBB's joinable groups feature, and makes managing the planning forum much MUCH easier on myself and our other web administrator, as planning committee members can add themselves to the department they're a part of right away, without having to wait for us to get around to their requests. While I grant that this is done in a limited context and for a forum with an intentionally minimal number of members, it does make managing the forum much simpler.
2) Specialized groups. I also run a fandom/wiki site. The wiki portion of the site, as well as other information-gathering aspects of the site's mission, are handled by volunteers who receive some elevated permissions and capabilities within the forum's wiki mod (currently running a hacked-to-pieces copy of Inviki). Anyone is free to join this volunteer group, but doing so requires them to send me a PM asking to be added to the group. Again, it's a relatively small and quiet site, so it's not a huge pain for me to manage this process, but it's a productivity drain, and a use case nonetheless, and one that I can't see as being quite as niche (or limited in potential scale) as the first one I discussed.
