Only drawback there is the normal ad issues. If you force the person to click the ad, that violates most ad systems. If you do not force the click, then you are dealing with the loew percentages that most ads get hit and a lower revenue level.
Yeah, and besides no one wants to view such ads that it is just a waste of time of some people. Not unless it was already a company or business with a well known name. And if I were that company, why would you still advertse your company or product if it is already known? But on the other side, captcha can help those bitcoiners who are only starting to earn. That's what I think.