Viewable is Futile!
While mainstream web publishers fret over the shiny new viewable impression measure, people involved in ad exchanges appear to be shrugging their shoulders. Because the exchanges price media based on demand, which is based on perceived effectiveness, many in the exchange crowd consider measures such as viewable impressions to be unnecessary data that is already baked into the pricing factor.
Another point brought up: due to various native formats becoming popular and other evolutions in the industry, this solution may come at a time when the problem might be on the way out anyhow. A comparison is made to ad verification, something people thought was a hot issue to address a few years ago, but which somewhat promptly went away with third party ad serving verification.
Whether the impression was actually viewable is a completely rational additional piece of information to desire if a media buyer purchases inventory in the traditional insertion order process, buying an arbitrary slate of impressions that may or may not actually be made viewable. A legitimate question, however, is whether or not a significant enough portion of online inventory will be purchased based on this method in the future, justifying the investment needed to distinguish among the different types of impressions.
Someone somewhere will probably wish to purchase all of the rejected impressions, so long as the methodology of detection allows them to be identified before they are exposed. Smart money would bet that those are monetized by packing them off wholesale and sending them over to the robots.