Well, most ads open in a new window. And most serious publishers barely have the resources to survive let alone do A/B testing. The publishers I work with thankfully shy away from network advertising and only deal with clients they have a direct relationship with. Regardless, it really isn't easy or very lucrative running a digital content based business. There are outliers, sure, but the long tail paints a much different story in my experience.
Whether or not one agrees with the outcome, if you have a large site you definitely should be doing independent testing. Google's ad reps, much less reps from other networks, will tell you to do things that harm your site.