It isn't clear what this means. Are you saying that radio isn't as valuable because we can't be sure that anyone is listening? That isn't the way that radio sells its advertising: "We charge less for ads because much of the time nobody is listening!" Beside the study that GP presumably references [0] specifically controlled for audience size.
No, I'm saying Spotify is more like owing the records -- you can play whatever you want, replay it as many times as you like, etc, than radio, which offers a lesser service.
So artists should get more from Spotify than they do for radio plays anyway.
[0] http://www.billboard.com/biz/articles/news/1155395/business-...