I'm not sure if or how this applies to online discussions, but in my experience in doing trainings and facilitating workshops, sometimes breaking people up into small groups for discussion can be more productive than having a single large discussion. But the value of small groups vs. everybody depends on the topic and the goals of the discussion.

I'm much less likely to comment on a post if I've seen that someone already brought up my point (or something close enough to my point), even if I might have expressed it differently and any resulting discussion if I had commented might have spun off in a different direction. So I could imagine that under some circumstances, consolidated comments might actually reduce the overall number of participants and diversity of perspectives. It's hard to assess, but I wouldn't be surprised if I was actually getting more participation in comments for at least certain kinds of posts, now that I'm cross-posting than before when I only had an LJ. But of course there are the tradeoffs that you mention.