Sure but that rules out most kinds of ads. Even contracting directly with clients to show their ads, the client is often going to want to run the ads through a

Sure but that rules out most kinds of ads. Even contracting directly with clients to show their ads, the client is often going to want to run the ads through a third party server so they can easily track all the different campaign they're running and check that the publisher is providing as many impressions as promised. I think basically you'd end up with a lot of sponsored content and "Brought to you by..." type sponsorships interwoven in the site so they cannot easily be blocked.