The only way I could see them implementing this is if it was a "default" option when put into a dock.
Having it when not charging/undocked would just eat up battery due to the screen always being on, and resources being used. For most people that'd be counter to the purpose of the phone. Plus what if you forget to turn that feature on, put on your music, and put your phone down and it just rips away at the life.
I could, though, see them implement this in a way that if you plug it into a dock and the dock is detected it enables the feature if the music player is on, or at least gives you the option "Would you like to enable Visualizer Playback?" type thing in the music player.