anonymous posts can very quickly change the atmosphere from thoughtful to spiteful, and it makes spamming/trolling much more difficult to stop. So you have to weigh those costs w/ the benefit of one fewer barrier to posting.
True, they're great for a site like 4chan. I think I'll just keep arguing for users only for now.
I'd argue for the opposite, allow anonymous in the beginning (during "bootstrap mode"), to minimize hurdles for new users, and switch to users only when anonymous users become a nuisance.
Just want to let you know, that I've implemented this.