Recommended reading: Fixed Fees and Diminishing Returns by Ross Rubin.

"Subscription music services have the most theoretical value to music enthusiasts. However, the bigger one's music collection is, the less incremental value one will find in the service. The ideal customer for these services is a mythical musical virgin whore -- someone who has a limitless appetite for music, but who never buys it. Napster-to-Go competes with Napster Lite, the company's more "traditional" online music store. As more tracks are purchased, the subscription value decreases."