Chatbox

Many forums have chatboxes in order to increase their traffic and their activity. Others administrators believe that a chatbox doesn't help a lot, because members will focus on the chatbox and not on the forum sections. What do you think? Do you believe that chatboxes are really helpful? Why?
Personally, I believe that they don't increase the traffic, but they decrease it. Members will probably talk with others via the chatbox without posting in the forum. FMSupport used to have a chatbox, but because of it, we didn't have much traffic.