I think Spotify is quite right in wanting to have only apps that add to the platform. Firstly – the biggest + for spotify over all other music platform is the ease in which they show you information. You search, you find – the official version!
It cannot have another experience in the app store – let’s say parker wants another lyric app which rights lyrics…. nice for the brand… but if they can only do that for 1 in 20 songs, it’s just irritating.

Other than that I think the apps are there primarily to make people listen to more music so they go over the 20 hour threshold – making them want to pay the € 5 per month.
Don’t forget that that is the biggest income of Spotify.