Spotify method is fine for their service. However I have understood that total ad revenue per user is roughly somewhere around 5-20$/month(?). This is for all the ad services total you use. So problem here is that currently many paid plans cost 5-10$/month/service (at very least). I think Spotify is worth what they ask and their ad plan is very under priced. However if every service ask 20-100x for paid plans VS ad plans idea about paying everything is not very attractive anymore and this won't work.

Also just like movies and games: if it's easier to pirate content than use it legally you are already lost. I may not to want to pay for every single services by credit card but I should be able to handle this ad-network scale easily.

Edit: Removed "I think" prefix from the comment since I THINK I repeat myself.