There's nothing wrong with placing ads on a site where there is nothing but ads. What if someone was looking for something? They click the ad.
If you are buying ads space from Google I'd be more concerned about their endorsement of people allowing their widget to blend into other people's sites. I was on a website a couple months ago where the person set up Adsense so nicely that it nearly fooled me into thinking it was part of their menu. If it had been your ad, I would have unwittingly clicked and you'd owe Google and the owner of that site money.