Sure there is, you start making the distinction between ads and regular content impossible. If all of the images on the site and all of the ads all map to somet

Sure there is, you start making the distinction between ads and regular content impossible. If all of the images on the site and all of the ads all map to something like www.sitename.com/resource/somekindofhash, then how would you tell programmatically whats an ad? You could even assign each visitor a mapping seed, so that caching still works. Blocking would have to be done on a user-by-user basis, turning this into a MAD-scenario, where some users just start blocking EVERYTHING that isn't textual or generated.