I own and run a fishing forum here in Australia and we have often had requests for a "chat room" and they are great but the big problem we saw with having one was the lose of valuable info that gets shared in the chatroom.
People will use the chatroom to ask questions and discuss topics but only those in chat at the time get to read the info and the info gets lost, yes they are greta for general socail interaction but can be a drain on valueable posts and info on a forum. They can also be very hard to moderate and can in some case cause arguments as there isnt the time for people that are having disagrements to think about their posts in chat before they post.
regards mick