I have it on my DJ board, if it were entirely up to me I wouldn't bother. A few regular members use it just to BS back and fourth/leave jokes and stuff like that so I maintain it for them.
My biggest forum has never been without a shoutbox. I don't like it much, but my members do. Can't say it has kept anyone from posting or anything like that.
When my forum was small, I had a shoutbox. But as more members joined, the system resources that the shoutbox used got to be too much. I finally had to disable the shoutbox.