I suspect that this isn't actually a big deal for most musicians and listeners, but probably indicates that Spotify wants to integrate their service into more peripheral contexts beyond basic headphones/speakers listening. Better compatibility with things like karaoke, DJs changing speed and pitch/Remixing, synchronized Stage/Lighting effects, etc. It isn't immediately obvious but the potential uses of making a music library that large MIDI-friendly actually could bring about lots of fun new applications development-wise. Shame that Spotify are one of the first to take the initiative on such a large scale because I'm not a subscriber, but this move could very well kick off a larger trend for squeezing more utility out of existing tunes. How these developments might play out in terms of labels and copyrights holders will be a different (and much less exciting) story.