Obviously the latter is more objectionable, but the big factors here are control and transparency. For example, I'm uncomfortable serving ads from large networ

Obviously the latter is more objectionable, but the big factors here are control and transparency. For example, I'm uncomfortable serving ads from large networks because I don't know what the end-user will see and because it's nearly impossible for me to know how my users are being tracked. If your proposed solution was open-sourced and gave me full control of user data, I think that would make all the difference. With existing solutions, company A tracks users and iframes company B in so they can get in on the action. And maybe company B wants to loop company C in, too? Many advertisers deliberately obscure these things because they're either unethical or part of their "secret sauce" -- it's less users having an issue with their view & click being recorded.