1. As soon as I'm playing more tracks than the number of tracks an average user pays I'm actually paying less than what Spotify currently pays.
2. People who do not play much music will probably not be exposed to much else than well know artists (Ie. Top 40). Or otherwise said: if I'm into music and spend time discovering not-so well known bands I'll probably play more tracks than the average user.
3. The number of people who will play less than the average number of tracks will be bigger than the number who play more.
The end result is that rich and well known artists will end up getting more money, and lesser known artists less.