When I am offline, I can still play songs from a list that is marked for download. But this only works, when I am already inside my playlist with the downloaded songs. When I tap on "my library", the first tab "playlists" opens, and the spotify app tries to download the list of my playlists.
This list of my playlists should be available offline, so that I can choose between several lists if I have more than one list made available offline.
Downloading / caching the list of playlists would also make the list load more quickly, because it is quite sluggish even on a good connection, and when on mobile connection on a local train, while the music keeps playing (obviously partially cached) every other part of the app loads slowly or not at all which makes the whole app hard to use.
So please make sure the lists of user's playlists (and probably some other parts of regularly used app content) is cached in future versions of the spotify app!
I use spotify as a native app on an android mobile phone.