The MRC is getting closer to lifting the advisory against viewable impressions as currency. You can download the PDF of their report here: Viewable
The larger question is what happens after the MRC steps away? At the joint publisher/agency council meeting in the spring there was broad consensus that viewable impressions was going to be currency however in the meanwhile the tools would be used a directional metric. Since then the tools have become broader in scope to address other issues as non-human traffic. Additionally some have developed a just in time delivery of the ad as the user scrolls to that area of the page.