The privacy angle looks like a non-issue to me.
Your public items get added to their news feed. They could view that information even if they didn't issue a friend request.
He makes a great point at the end about beefing up the network effect and he is right. This is just another way to pump more noise into peoples' feeds.
Today Facebook is really asking for bad press about its default user settings. This just made it to the BBC: <a href="http://news.ycombinator.com/item?id=1711973" rel="nofollow">http://news.ycombinator.com/item?id=1711973</a>