I agree that comprehensive catalogues matter more for music, but I think we're missing another area where Netflix and Spotify are similar (and where Spotify may arguably be better than Netflix): acting as a recommendation engine.

Netflix has been able to track exactly what types of movies consumers are into so that it can deliver more in the same vein. Originally, it would send consumers to other movies it licensed. Now, as a producer, it can direct consumers towards its own flow of productions.

Spotify's recommendation engine has long been a draw for many users. I believe Spotify is setting itself up to be able to increasingly direct users into its own Spotify-flow of artists and musicians. The recent change to playlist mechanics (so that playlists autoplay "recommended similar songs" once completed) will make this even easier.

Having a huge catalog is important, but controlling the flow of what's popular may trump the back catalog in the long run.

edit: Spotify is not as far along in the process as Netflix, obviously, and the entrenched nature of the music business doesn't help, especially with the market practice of locking artists into 7-year exclusive contracts. I believe it will go in the same direction - it's just going to take a little longer.