Discovery of independent music, as well as remixes.
I have a huge SoundCloud playlist that I keep adding to.
I also use Spotify. Another huge playlist. More music I like.
I hate having to choose when I really want to mix the two playlists.
Edit: I see that API still exists at the moment. I wonder how that works with the ads they have running on the website. Do you currently get no ads if you use a custom player which uses that API?