Ordinary captchas exclude blind and other visually-impaired people. Audio captchas exclude deaf and hard-of-hearing people. Clickcha excludes people with muscle-control disabilities.
Of course, there would be alternatives for people with disabilities.
It suffers from the same issues of forcing commentators to stop and think about something utterly unrelated to the comment they are trying to make, and of being easily defeated by third world workers paid a pittance or first world porn addicts.