So I decided to give Spotify a go a while back, and so far have been enjoying it. However, it really won't do to part ways with the best visualizer in existence, Milkdrop.
Instead of disqualifying Spotify as a music player option due to its lack of support for visualization, I decided to solve the puzzle of mapping the sound such that I can play music with Spotify and use Milkdrop to visualize it.
Here's how I did it:
1. Install Milkdrop + line-in plug-ins for Winamp
2. Mess with your soundcard until you can get the Stereo Mix to listen to computer sounds
3. Set the soundcard's Stereo Mix as the default recording device
4. Play music with Spotify, then open Winamp and add the URL 'linein://' and play the 'song', then start milkdrop