Some people are subscribed to Telecoms with data capping. The chatbox consumes a lot of data so it will be harmful to the internet users who have data capping limits on their internet connection. Chatbox is okay if all of your member shave fast internet connection. For sure some members can enjoy the Chatbox and have live conversations with people but think about those people with Data Capping, Are you willing to have chatbox on their expensive and give them a slow internet connection because your chatbox consumed a lot of data.
I don't think chat option is necessary. In my opinion, every kind of discussions should be in the threads and posts and not in the private chat sections. If the discussions happens privately, it will not help the forum or a website. Private messaging system is better than chat system.
I agree with those who say chat boxes kill a forum's activity. If someone wanted to chat they can just go join a chatroom. The forum is about posting topics and replying that way. I do not have a chat box on my forum and will never get a chatbox for the forum.
I think it's a good idea to add one the forums or blogs, like you mention it will keep users on the site for a longer time. Maybe this will keep them entertained while waiting for a topic they are interested in if they can't seem to find one at the moments. Besides it gives the opportunity to interact with others directly.
I believe that a chat box should only be limited to premium, veteran, or staff members, or publicly once a forum is off its feet and has active members. A chat box does hurt activity, I have witnessed it in the past. Most members love chatting, and having a quick way to chat prevents them from making content and talking through posts and threads.
If you have a new forum, it will certainly hurt the forum as it can take activity away on the forum. Once you forum is established and has a fair amount of activity, then I can see it being okay as long as that activity doesn't go down.
There is one forum that I used to be an active member. It has a chatbox that is very dynamic that sometimes the community seems to forget the forum boards in favor of the chat box. And if the chatbox increases the hits then I would say it's good for the site. Anything that solicits action in the site is beneficial and should be considered an asset.
I think it's a bad idea because it takes the activity away from the forum because a chat box is easier to use than the forum. You don't have to click a link to get on the chat box you just type stuff into it. It saves people time from typing in a forum thread. I personally hate them with a passion, I've seen a lot of forums die in the past because they had a chatbox and when the admins tried to remove it, there was a lot of trouble.