Was just talking about this in another thread: I could never give up the flexibility of a subscription plan that lets me pick and choose exactly from a giant catalog of music, not just what I own. I decided on rdio over Spotify in the end.... use it *constantly*!
It took me a while to learn how to take advantage of having all those songs available, but once my music listening habits changed (less listening to a set of favorites, more constant exploration) that was it for me.