The option to allow anyone to register to your WordPress blog is basically useless and should be removed. The reason why I am saying this is that once the automated spambots find your blog, they start registering users in the hopes of using your blog as a spam distribution platform.
You can use CAPTCHAs as a gatekeeper to your registration dialogs. But the fact alone that this is necessary angers me. Just because a bunch of rightfully underpaid software engineering dropouts thinks it is a good idea to support the spam industry, the rest of us have a harder and harder time using the Internet for something useful.