@hilbert,
It's a personal choice, Hilbert. Try clicking on the "thumbs down" button beside any thread. It'll disappear from view for you and it'll be replaced by "Topic voted down (view)".
Drag your cursor over it and you'll see which thread it is. Click on 'view' and it'll come back into view - I think, until you refresh your screen or maybe, it then stays in your view.
EDIT: I just checked. Clicking on "view" just brings it back until the screen is refreshed. In order to bring back a "Topic voted down" you have to give it a thumbs up. I believe that only one thumbs up per person is allowed, so if you had previously given one it seems logical that you could never bring a Tvd back to permanent view.
2nd EDIT: You can indeed thumbs up a thread as many times as you want, but they don't appear to be permanent.