I agree with this - I think a lot of people lurk on HN (like me) and only join when they've got something worthwhile to say. It's when it's all take and no give that it becomes a problem.
But those people are "giving" with their vote and the people who don't register are "taking" albeit taking something pg is clearly willing to give (bandwidth). So when all you have to do to give back to the community is create a password and click the mouse on a few arrows why wouldn't you.