I think in the end it helps way more than it hurts. For every batch that goes quickly or HITs that get snapped up right away there's also a ton of other ones I would've never found out without the forum.
Maybe it could hurt the very experienced turkers who know what they're doing and how to find good work, but if they're very experienced then they also probably have steady work to do, closed quals, etc.
And besides, it could never be helped really. If you had a forum where people kept things close to their vest and HITs weren't widely shared, people would just bolt and go to another one and make more money there and the end result would be the same.