Trolls are important.
Trolling, especially as seen on places like slashdot/4chan/somethingawful/etc., can oftentimes be a mechanism of critique for ideas and rhetorical styles.
One of the best things about the 'net is that, frankly, none of this really matters. None of it. It's a big joke. My twitters and my wikis and my posts don't mean anything. They're bits in the stream. My karma is an int on a server somewhere, incremented and decremented by the whims of my fellow users.
Trolls can help remind all of us that hey, this is all light-hearted. They say outrageous things, they stir up trouble, they cause annoyance, they sully the pristine conditions of these high-minded realms of discourse.
In short, folks, they keep us all honest. They call us on our bullshit. And when a community takes itself so seriously that it becomes a habitat for trolls, it usually is a sign that that community needs to be dispersed, cleaned, and reformed elsewhere.
HN is a pretty cool place, and I hope it lasts a long while before ossifying and becoming infested with trolls.