Yes, groups seem a bit odd, since many of them could probably be as easily covered in the regular forums (Pinhole for an example). I do belong to a half dozen, but not much happens. But I suppose for really small, off-the-wall specialties, they could help things stay organized.
My biggest question about the groups is what exactly does "Last activity" mean?! Maybe it's as silly as someone just looked at the posts, but several times when I've seen a recent time stamp there, I have been unable to find anything that has changed within months. As though people look, say "nothing to see here" and go away, but it counts as activity -- not a very useful function IMHO.