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There are two classes of bots, spamming and content stealers (monitors). Radian6 in your home country makes a living using bots to steal other sites content for its clients, some which get pretty low and slimy in the food chain.
My focus was on the content stealers. Human assistance during registration would allow bots (pre-programmed to use the auth) to autonomously login as a user through a captchaless login process. While your suggestion is great for a spam bot, the same could not be imposed on bot that reads content without the same having to be imposed on the visitor. That would be more a pain in the dupa (doopa?) than a login captcha.
A login captcha, if it existed would have been an incrementally better because the bot would not be able to log in without human assistance. And I am not asking anyone to develop one, I was just wondering if one was already available.
Do you think that I should change the wrench in my avatar to a sledgehammer instead?