Are CAPTCHA's a good idea?

CAPTCHAs aren't very good for keeping bots off websites.

reCAPTCHAs are very good at getting people to volunteer there time to digitize books.

I always purposely screw up on the OCR part of the reCAPTCHAs because I don't like being used for OCR.

It seems to me that the word that isn't curved is the one from the scanned book and isn't validated to check the CAPTCHAs, but instead used to write the book which I hopefully screw up. https://www.google.com/recaptcha/learnmore