We originally had this, and also users viewing the forum. I believe it was removed either a) automatically as part of our routine software upgrades, with the new version defaulting to having it off, or B) conciously by hando to reduce server load.
I honestly don't think it adds much weight to the server so maybe hando could consider adding it. I found it fairly useful when in a discussion with others, knowing whether they are likely to post or not in the next few minutes.