As a trivial side-note: When encountered with a ReCAPTCHA, I'll fill out one of the words and put in gibberish (or other text) for the other. For some reason I find it satisfying to "pull a fast one" on any captcha service.
Why? What do you achieve except being told you're wrong every once in a while? I mean, you're trying to bother a machine. Isn't that a bit like a reverse Turing test?
I suspect the amount of noise that ReCAPTCHA filters out from automatic attempts is several orders of magnitude larger than anything any group of actual humans can generate.
As far as what satisfaction I gain from it... I suppose I find it to be a sort of rebellious act. Also, I did not get accepted into Carnegie Mellon.