You couldn't listen to any song and as such it didn't eliminate the need to buy (or pirate) some tracks. Certain record companies had yet to sign on and quite a few major bands were missing from the database. Bands such as Pink Floyd, Led Zeppelin, The Beatles and ACDC (there are numerous others, these are just the bands I like which were missing and really stuck out to me).
I was a paying customer for around 9 months using the service to sync offline playlists with my Android phone (wirelessly, as there were no other solutions that allowed me to do this easily), but I canceled my subscription when they failed to acquire more of the artists I liked and other solutions came along to allow me to wirelessly sync playlists to my device.
There are better ways to handle your online music now. Spotify had a real chance at becoming top dog, but in my eyes, they didn't innovate and provide the features the public wanted quickly enough.