Ask any foreground radio personality who had great ratings in diaries and then suffered the all-too-common precipitous drop in their first month of PPM measurement. Just because listeners to your station like you enough to write your name in their book does not guarantee that they listen to you at all in reality.
When you break down the individual media vehicles, time spent is actually quite easy to report on -- so why shouldn't this become the de-facto metric for measuring engagement? If consumers spend more time than the average, they must be interested in your product or service -- and if they are interested, that is a measure of intent. The more efficient you are at implementing a campaign, which includes paid as well as non-paid placements, the more likely your campaign will be to drive that increase, which ultimately results in sales.
How engaging, entertaining and expeditious is your content? It's not how long you talk. It's how long they listen.