The HN community wouldn't like it (to put it mildly) if we deleted things based on what we consider unnecessary. They're a curious bunch who want to be able to see everything and decide for themselves. On the other hand, most users don't actually want to wade through the dreck; they just want the option.

The solution is: we don't delete comments outright unless the author asks us to. The most we'll do is kill them, which means they get marked as [dead] and are visible to users with 'showdead' set to 'yes' in their profile.