If you listened to less music than the average person then some of your fee went to the people who listened to more music, and the other way round if you listened to more music. Which is going to average out in the end anyway, while massively simplifying the accounting. Spotify can also tell artists exactly how much they are getting paid rather than having to wait for the end of the billing period to work it out. Only to come to roughly the same amount anyway.

There is no tracing routes with a pooled fund. Only inputs and outputs. And the outputs would seem to be pretty much identical in the pooled system vs individual pools per user.