I've been a member of the SE for a few months now and I've noticed that usually the worst quality answers come from unregistered users.
There is no point on down-voting since the unregister user won't be bother for their lost reputation.
Sure, we flag them, and eventually the answer will disappear, but shouldn't we avoid all the trouble?
Questions from unregistered users sometimes are good, sometimes are bad, but there is more bad than good coming from unregistered users answers.
Almost every other internet site requires you to register and login to participate or post anything. If you want to make contributions to a community why wouldn't you want to be a member?
Edit: As pointed out by fedorqui "solo el eco me responde" :-) but there's been some discusion about this on META "Post as guest" is a bad user experience and encourages low quality contributions. Get rid of it
If you can't comment until you have some reputation, why can you ask and answer?