Not a big deal, but if you're trying to learn a piece, it's nice to slow it down and cycle through a specific section. There are apps to do this with music you own, but not streamed music from spotify.
Idea has been reposted here: https://community.spotify.com/t5/Idea-Submissions/All-Platforms-Other-Allow-users-to-alter-playback-...
Updated on 2017-08-24
Hey folks. We're marking this idea as Case Closed today as it's "Not Right Now" status hasn't changed in over 3 years. We're working hard to update the ideas you're voting on that have a chance at getting built at Spotify--we're trying to be fully transparent here. Please continue to leave your feedback here in the Idea Exchange so we can bring it to our product owners, thanks!