The main flaw in this plan is the context in which Captcha's are generally used -- sign-up forms, comment forms etc.

They're part of a deliberate user action, not part of a general browsing behaviour.

People mid-stream in a sign-up form are very unlikely to follow an ad -- or if they do so, it might be at the cost of the primary activity.

I think it's for reasons similar to this that Facebook ads perform so poorly (apparently).