The issue is if the site has very few ads and no adult content, it probably doesn't get much advertising revenues so the day they are purchased by another company or start losing money they'll start putting naughty content in their ads.
That and not many people know it so they might be scared of clicking the weird links (it even happened with adfly), so while the person using this service might be getting extra revenues, it's possible he might lose traffic too until the website becomes mainstream and well known.
