Chat rooms detract from the forum as the conversations that would take place in threads are being addressed in the chat room. also chat rooms require constant moderation as things can and usually do get out of hand. especially when open to guests. There also needs to be constraints and a well organized set of rules and guidelines in place.
In most of the chat rooms I have been involved, many of us ended up exchanging phone #s and conversing vocally. much more enjoyable than the endless typing that took 10x longer to say what you wanted.