first off, i dont really like CTR as a measurement model. It assumes that every ad is based on conversion/acquisition and places zero value on the actual brand value of engagement. it also assumes that ad sizes will remain the size of MPUs and Banners forever.
Beyond ranting about the obvious (CTR pushes all of the risk onto the publisher and the advertiser can do crappy ads), without a large enough and efficient enough bidding environment aka google, you cannot manage your price point effectively to make it worth your while, reducing your client base significantly. The emergence of click to call from google is another cunning entry into that marketplace, where you can call a company straight from your browser.
Impression based advertising is already leaps and bounds ahead of our traditional ad measurement models (did you really see all the ads in the paper?) so why does the ad publishing industry now focus on even more complications in this regard? I think share of voice is the next important premium metric to really focus on and reducing ad spots per page but increasing their size the BIG next step… why?
move away from acquisition to branding based advertising
provide better impact for advertisers, less cluttered experience for viewers
increase scarcity, increase value