In an effort to fight spam, many systems that are openly editable have opted for a simple captcha to verify the humanity of editors. Instiki has recently been targeted by spammers as well, and for those places that all contributors don’t necessarily have an account, captcha is an excellent choice for keeping your wiki open, but spam-bot resistant.
I’m currently working on a patch for the current version (0.11.0) and I hope to get it uploaded to the Trac soon. If it doesn’t get accepted into the core distribution, I will put the instructions for getting this quickly enabled up here.