You know a peculiar thing that humans do? They do things slowly. You know what bot (programmers) hate doing? Doing things slowly. When you've got bots that are diving through thousands of registration forms per minute, and suddenly you need to slow them down to 15 seconds per form, well, that's already an enormous win for the site owners, even if it makes the new captcha fail (if the quoted statistic of 99.8% of captchas being solvable by bots is correct).