All the time I leave the wifi connection and enter the mobile network (which my phone does automatically when no wifi is available) I have to manually activate the Offline mode in Spotify in order to save my mobile quota for other apps. Why can't they simply add an option in the settings menu to go automatically in Offline mode when the phone is not connected to a wifi network?

There would be an even better improvement: An option where you can set "stream music using wifi networks only". Streaming is the activity which costs a lot of quota but a simple search query when you are looking for a song doesn't effect the quota a lot. So that can be done mobile.