My advice is to be selective with your advertising partners. It still seems like an odd place to have ads, but if they were more tasteful or I had a say in what they were, I might be more inclined to use it. That said, it strikes me as a very odd revenue stream. If it were me, I would concentrate on making this the captcha of choice and then focus on monetization. The ads seem like they might kill off any chance of this happening.