by this argument, the captcha would become useless as soon as it saw widespread adoption. The idea of a captcha is to be strong against bots even if it was directly targeted by spammers.
The problem is to design a CAPTCHA that implements just enough headache to make it worthwhile not to overcome and at the same time not frustrating users. I think this chess problem uniquely and elegantly solves the problem for the site in question by achieving both.
Then again, I am not familiar with the users, maybe the site is often trolled by chess mastah wanna-bes.
http://www.codinghorror.com/blog/2008/03/captcha-is-dead-lon...
If every forum had a completely different captcha system, forum spam would pretty much die out.