The captcha stops the spam bots. Also, new members are required to activate his account. If you haven't needed to do it, it's because I know you.
Lately, there has been an inordinate amount of, probably hired, Russian spammers.
There is also the problem, that you don't have, of the pirates. If you allow nicknames, there is no way of knowing if he is a licensed user using a nickname or a pirate.