I find I constantly refer to my "My recent posts" tab to keep track of responses to questions, comments, and so forth. Extremely useful. Like others, though, I am guilty of posting a useless comment to a thread (i.e. "just tracking this!") in order to keep track of an important thread.
When lots of users do this over time, it means lots of threads indicate they have new comments available, but the new comment turns out to add nothing useful to the discussion. For trackers, this means a few wasted minutes each time we go to check out a thread where this has occurred.
Absent a better method, I don't really mind people using the "just tracking" comment method to follow a thread. (And absent a better method, I will continue to do so myself. But given Drupal's potential (and the number of Drupal.org users), isn't it silly that, as far as I can tell, there is no other easy, silent method for tracking forum threads?
Issues queues allow us to subscribe via email, but this is actually not very useful. What I would like to see is something like a "track this" link that allows users simply to click on it, and then refer in a tabbed window to a list of tracked posts like the "My recent posts," indicating which has been updated. This could be a "My tracked posts" tab right next to "My recent posts."