Still on the side of diving, having heard the panel.
1. Many of the anti arguments come down to insularity of the movement, as I suggested in the first post. I.e. it's weird if you run into a former significant other at too many of the same social events. This can be a problem, but can also be a problem if you try to date someone from an insular social group of high school, college, or grad/professional school friends. The alternatives include dating people that one meets randomly in bars or coffee shops, but there it's easier to run into either a loon or someone with whom one lacks any common interests without knowing it. There's also the various internet services, which I admit I never tried, but again, it seems like it's easy that way to rack up a lot of brief dates with no real return.
2. One panelist mentioned "girl detente," i.e. the tensions that one runs into with another female if you've both dated the same guy. Yes, this can happen. On the other hand, potential for the same problem exists if one dates within a group of non-movement friends; see above.
3. For women, there is the social climbing problem -- dating someone who's much older or more experienced within the movement can be seen as an attempt at social climbing and thus reflect badly on the woman. Again, I suppose this is a possibility, but the more narrow solution seems to be "avoid situations where there are really big age/power imbalances." I would hope that most women also have a sixth sense for being able to identify That Guy who preys on interns like so much fresh meat. Those That Guys are usually identifiable from a mile away.
4. Of course, one should also try other strategies for finding a partner. Insisting on dating exclusively within the movement is probably a bad strategy. But ruling it out pre-emptively is also silly.