I think such things are actually good.
When you get a bunch of people together for interest x, some of them are going to naturally be interested in other things in common. In a group of 50 martial arts students, surely more than one of them knits, enjoys anime, watches American Idol, is a member of the SCA, does hatha yoga, and so forth. There are two ways they can address this mutual extracurricular interest. They can either spend free time in class (when chatting is permitted) chatting about this other interest, which I think would really alienate the other students, or they can meet away from the group to indulge in their mutual other interest.
I've been involved with splinter groups like this and while I wouldn't call the splinter groups CIA eyes-only secret, I have found that it's better not to discuss them around other members of the larger group. Because some people who don't like other interest want to be included because it's a meeting of club members. The fact that it's a gathering of club members who also enjoy other interest ABOUT other interest does not reach them. They want to be included. And then they are the wet blanket wanting to know why we always have to go to the Irish pub and can't go to the techno club when everyone else is a Celtic music fan; or why do we have to watch another horror movie, can't we watch Shakespeare in Love, and so on.
(I don't entirely fault people for wanting to be included. I feel a bit left out when people in my knitting circle whom I like go off to a spinning retreat. The fact that I don't spin and don't want to learn how and have publicly stated that I don't need or want another expensive yarn-related hobby makes me feel a bit chagrined that I have these feelings of being abandoned. But I recognize that that is *my* issue. Not everyone is willing to admit that it's OK for their friends to have other friends and other interests.)
I remember very well being at my knitting group and discovering that another person was a Connie Willis fan. We went off about it for at least twenty minutes, totally preventing any other conversations from starting in our area of the room. And then we ganged up on this other woman who had no idea what we were talking about: "OK, you *have* to read 'To Say Nothing of the Dog.' It's imperative." "But shouldn't she read 'Firewatch' first?" "No, 'Firewatch' is in a book of short stories. They'd be distracting." "No, read The Domesday Book.' It's shorter" "It's so depressing, though. 'To Say Nothing of the Dog' is a love story, see, set in both the future and the past, and..." "But you have to know, since you like cats, that there's been feline distemper that's killed them all." "But there are cats in the book, in the past part." At the time, I felt like I was high, and it was a gleeful, joyous thing. Looking back, we were being really rude to everyone sitting around us, and probably obnoxious as well. We would have been much better off retiring to a pub afterward. At least Emily forgives us, as I lent her my copy of Bellwether, and she really liked it.