There are a couple issues with your current registration system, mainly pertaining to security:
1. There is nothing preventing you from registering multiple nicknames with the same email address.
2. There is no confirmation email of any kind sent after registration. (This isn't as serious as the other issue and not exactly a bug but may be something you wish to look into.)
This post isn't misplaced, we just think that a captcha security isn't so important for the moment.
About multi-account, it's not automatically prevented but we have something against it.