Exactly. The whole discussion here points to the major flaws in using text as CAPTCHA. Maybe if questions were more metaphorical they would be harder to guess, but then there is no absolute answer. I think a combination of text, image, and logic would be hardest to break.

It's still a game of guesswork. Generally if you fail the CAPTCHA you will be offered a new one; and any good system should lock you out after a certain amount of failures.

Assume spammers are using botnets. The problem of locking them out is as hard as detecting them in the first place.