Spotify has a few nifty features that must be using a fair bit of bandwidth. Features such as the ability to remote control another instance means maintaining a control channel. The ability to switch music to your local device means all devices must stream the song, as though it were playing on that device, to allow immediate switching.

These work quite well for the user, as it's enjoyable to listen to music on your stereo at home, then just tell the phone app to switch from your stereo to your phone (maybe over headphones) while you're mobile. And heavy use of P2P between devices on the same LAN or Wifi network would reduce internet bandwidth use, but may unintentionally cause a lot of churn and disk use.