Well, yes and no. It's not expected that streaming services will open their APIs, but as more and more have made their internal mechanisms available for creative plundering, it could be considered unwise not to. Spotify was one of the first music streaming services to open up its API to external developers in 2011, partnering with music editorial platforms like Billboard, Rolling Stone, and Pitchfork to create company-specific apps; 2012 saw 12 new apps from record labels, including Universal and Warner Music Group.
It was a move that distinguished Spotify from competitors like Rdio, MOG, Rhapsody, and Google Music -- that is, until the next year, when Deezer made its own API available. Not to be outdone, in 2013 Spotify integrated with the Echo Nest's API, giving developers access to the music intelligence platform's enormously influential algorithms (recently, the company updated its web API, allowing access to user profiles and metadata for more specifically customized playlists and more extensive album, track, and artist details).
So yes, it makes sense that Xbox Music, which launched in 2012 following the crash and burn of Microsoft's Zune mobile streaming service, would follow in those steps. In an increasingly crowded streaming market, one way platforms differentiate themselves from each other is through user-created apps, many of which increase accessibility and usabilty of the platform itself. Alexandre Passant's Long Tail, for example, lets users deep-dive lesser-known tracks from artists they already know and like.
As Billboard's Glenn Peoples pointed out, "No single music service has the resources to create all the apps that will enhance the underlying music service and keep a wide range of music fans engaged and interested."
And so: Enter the hive mind.