One thing that iTunes does not do very well is give new music recommendations.The problem with the iTunes remora model is that you survive at the whim of Apple. At any moment, Apple could eliminate you by simply launching a better built-in recommender service in iTunes.
Poor recommendations has driven a whole industry of music recommenders that work with iTunes.
These recommenders run as iTunes plugins, sidebars and tag-alongs. When you start iTunes, these programs also start up - they keep tabs on what you are listening to, and use this data to generate recommendations for you.
These recommenders are like remoras (aka suckerfish) to the iTunes shark.
By attaching itself to a host such as a shark, a remora benefits by using the host as transport and protection and also feeds on materials dropped by the host. The recommender remoras benefit from iTunes reach onto millions of desktops and feed on the opportunities left by iTunes' poor recommender.
However, Paul may be overstating the case a bit. Services such as iLike and Last.fm have iTunes plugins, but they also have independent websites and applications.
In fact, given that Last.fm has plug-ins for Launchcast, Winamp, Windows Media Player, Skype, and many others, I suspect these recommender services are more using the plug-ins as a marketing tool to drive traffic to their service, not as their primary means of survival. I would be curious to see numbers on how much of iLike and Last.fm usage is through iTunes so I could know more.
On a broader issue, it is concerning that using open APIs to integrate to other services and applications may create a dependency. Integration is fine if it merely increases the visibility and convenience of using your service.
But, if it gets to the point that you become a remora, you may find your survival subject to the whims of your host.