The big problem with using media measurement to gauge media consumption is that we can really only measures access, not consumption. The advertising world is built around impressions for that reason; there's an implicit assumption that some percentage of "impressions" will go unnoticed. Measuring that and trying to get your ads in a format where they will be consumed more often when they are shown is pretty much the focus of the entire advertising industry. For a very concrete version of this, think of how DVR users skipping commercials has led to an increase in product placement within shows.
Agreed that measuring consumption is tough - even if it's as simple as measuring an ad view, ad click, and then a purchase you need to take into account everything the customer has seen before. No way to currently measure something like that.
We're going to be stuck in perpetuity with John Wanamaker's: "Half the money I spend on advertising is wasted; the trouble is I don't know which half."