It wouldn't provide any meaningful security; think of how difficult it is to make a CAPTCHA that is both readable for humans and hard to beat for computers. Add

It wouldn't provide any meaningful security; think of how difficult it is to make a CAPTCHA that is both readable for humans and hard to beat for computers. Adding security to a conversation is a solved problem in the technical sense (use encryption such as GPG), the only reason that it's not in widespread use is that people can't be bothered.