I think usergroups also can help the forum by potentially giving its members a stronger sense of belonging. If a member of RPGWatch is a member of a usergroup he may be inclined or have a stronger interest in visiting the site more frequently, and thereby might post in public forums more often. I'm just speaking from personal experience though. When I was a member of the usergroups back when we were all proud members of RPGDot, I found that I was frequently reading/checking the posts at the usergroup and often times figured... 'well, I'm at RPGDot so I might as well surf the Gothic public forums as well while I'm already here.' Yes, some people might just go to the forum to post exclusively in their usergroup, but I think most people will at least usually be distracted by the convenience of the public forums to such an extent that they'll also visit them [more frequently] as well.
I also think usergroups are a good idea because it hides specific interests or forums that aren't directly related to the main objective of RPGWatch, which (I think) is to provide a source or virtual home for RPG gamers to seek and share knowledge in a friendly and mature atmosphere. If I visited the main forum index and saw a lot of forums for things not directly related to any specific game, such as politics, sports, programming, hardware, etc, it'd dramatically alter the atmosphere here and needless to say that'd be misleading and it'd lose its personal connection with the viewers. The public forums adhere to a fairly strict topic; RPG gaming. Usergroups have the luxery of hiding those unfitting distractions (the topics that don't apply to the overwhelming majority) while at the very same time remaining loyal to the objective and caring enough for its members to allow places exclusively for those who share the same interests that are outside the majority.
Anyway, these are just my thoughts on the subject. I don't presume to represent the majority of community members. This is just what my perspective towards usergroups has evolved into over the many years whilst with you all at RPGDot.