It is also in the consumer’s interest. Just yesterday my friend Kirk found some new music because he follows me on Rdio. But I can’t do the same thing with my friends who are on Spotify. Because all of these services are silos, by definition of their paid business model. If a roaming network existed, there would be more social music discovery, listening, and, I believe, uptake of the paid subscription model by consumers.
If Apple ever does a subscription service, I think most people would default to using what they offer. Considering Apple is the most popular music distributor in the world, and if most people used their service, it seems likely it would turn into just what Fred described — except all on one service.