Your phone has an app store, your computer has an app store, your web browser has an app store, your music streaming app (which you found in an app store) has an app store. I don't know where you would define 'the line' to be here; but I feel it has been crossed.
So Spotify announced at it's "what's next" event in New York that they will be supporting app development on their platform. Not going to even talk about the fact that they haven't stopped forcing you to login in with Facebook; but something like this just seems insane to me. Don't take this as an attack on the rich development community that is being bred through the thick mixture of both open and curated app communities we have; but to the consumer-side, the over-crowding of app platforms means most of these surely aren't going to be used.
In some locations, a platform for apps makes sense. The smartphone is a prime example: taking the constricting world of mobile downloads as they were before (choked by your carrier) and expanding it so that your phone can become a central consumption device. Facebook apps (no matter how much they annoy me) work: they increase your internet usage compatability with the social network, and it livens up a network that would normally look rather plain (even though I'd prefer plain; but we're looking at this from a different point of view than my own).
However, a Spotify app platform, presenting a sort of Inception-esque app store within an app store mentality, is something that is entirely pointless, and won't be used beyond that few minutes worth of interest when you see a banner announcing it. We went too far with apps on TV sets.
Step back, take a breath, and focus on what your service is for...listening to music. No matter how well they dress it (I'll even admit some of the ideas look pretty cool), you just can't seem to escape the thought that maybe you'll just carry on as always: listening to you Spotify playlist in a minimised window. It's almost always a background running application (the same as music is usually a background to your lifestyle), don't try to bring it to the forefront of attention in such a needless fashion.