There is a danger that if you make your content look too similar to your ads then it will annoy people. If you have a site that doesn't rely on return visitors then it's going to be a plus.
But ... does this really affect sites that expect to have returning visitors? If they visit sites so often, wouldn't they start kind of blocking out ads in their minds? Ok, if ads are as intrusive as pop-up and pop-under, then there is a problem, but can contextual ads become so annoying?