Well, it is a good way to tell computers and humans apart. Computers can read it, while humans can't. They should reject corectly filled in captchas.
I still loathe reCAPTCHA more than any other captcha out there. I've given up hopes of logging in on many occasions simply because of it. Looks like it's designed for not to be deciphered either by humans or computers.
You actually don't have to get the latter one anything close to right; and I think you're given the option of at least one typo in the actual CAPTCHA test.
So when a CAPTCHA looks impossible, try doing the possible half and typing in "balls" for the other half. It actually works with pretty reasonable accuracy.