Exchanges keep trollboxes because it's fun. For example, being in Poloniex watching charts for an hour alone would be boring as hell without the trollbox. I enjoy chating with random guys and having some guy with some subtle trolling. Sometimes you can find a legit conversation in between all the trolling. Sometimes predictions are correct and sometimes you get scammed (not me since I follow my own agenda I trust none of the predictions). But overall is a good idea, it brings more traffic and fun.
So it provides a kind of communication that bitcointalk doesn't provide.
But I see people in the trollboxes inviting each other to irc or slack or skype, because trollboxes aren't suited for in-depth conversation and working relationships.
I just think instead of hundreds of isolated irc chats, it would be good to have a big place where people can get together that doesn't scroll by and disappear in a few minutes.