I downvote my fair share of nodes, almost exclusively because they are abusive or dismissive. While these nodes are relatively uncommon, they do still get posted, and by downvoting them I am doing my own small part to help "clean" the site. I might also be tempted to downvote a completely offtopic or clueless node, but only if I'm fairly sure the fault is deliberate. I don't feel the need to punish someone for breaking "the rules" unawares, unless "the rules" are those of human decency and respect.

As for why I don't use these points to upvote other, worthy nodes... There's no filtering on this site, so unlike on slashdot, even nodes with no votes will still be seen -- everyone has a voice. So while upvotes only help the author of the node (by giving him/her karma), downvotes more directly help the site as a whole.

/s