I'm not sure the interface for Spotify works quite the way I would expect.
If you have other screens up (e.g. Navigation) and do a voice search for Spotify, a pop-up window appears listing matches (artists, albums, songs and playlists). If you tap on one of these, nothing happens! You have to navigate into Spotify to see the results and then tap again to play a track.
My expectation would be the artist, album, song or playlist starts playing when you tap on it. I thought Rdio used to work that way.