The economics of spam commenting makes sense if it can be done automatically at a massive scale. If only one or a very small number of "humanity tests" exist, then cracking those tests has a high payoff value. Having a huge number of different, ad-hoc authentication schemes would make developing automatic cracks unacceptably expensive for the actual level of benefit achieved by posting the spam comment. Therefore, being a site with a unique CAPTCHA system attacks the core value proposition of spam comments, which strikes the root of the issue.

Another way to look at it is like the entire collection of CAPTCHAs on the Internet is really just one big CAPTCHA library, which tests a subset of "human" abilities. The larger the subset, the more difficult it will be to circumvent. Relying on one or two special abilities makes it easier for a machine to emulate those behaviors and gain access. I hope that clarifies the point I was making.