A section on the threads that just lets you add yourself as "I was here" to be a part of the discussion may clean up the real thread for people who truly have something meaningful to say.
In essence the (this thread has been monitored by X users listing panel)
With a list of people observing the thread and their names in a list
(Sort of like the Who's Online feature that some boards use)
To answer your question in my opinion it adds a signficant bloat to the page, as there are a lot of users here a +1 function could do the job but then it needs to save the list of users if you want to find your own.
So while it does seem interesting it would be impractical to use for the utility you get from it.