I don't really either. But I lurk anyway, because then I'm caught-up on all the discussions and can jump in when I'm taking a break from coding.
Personally, I like chatrooms because they're much better for networking. I'm much more inclined to visit the startup of someone I converse with (as opposed to comment with), I'm more likely to make introductions for them, recommend them for jobs, etc. Basically, I could consider someone that I spend a fair amount of time chatting with a "friend", while someone who I only talk to on message boards is a "professional acquaintance".
This is mostly because of everything you dislike about chatrooms: they're disorganized, they tend to veer off-topic, they devolve into social chitchat, they're spontaneous, and they take up lots of time. You don't really know a person until you see how they behave spontaneously, when they aren't thinking out their response. Hell, I probably seem significantly less intelligent in chats than on here. But it's that vulnerability that makes me trust folks I chat with real-time more than folks I discuss with on a message board.
BTW, I'm also in #news.yc on EFnet, along with Jey. Kinda quiet there at the moment (I think we're both working on startups), but that's just because it hasn't reached critical mass.