I actually thought of a rather interesting issue they have going forward with Brawl ports.
So on the store page for Pirate Ship, it actually mentions that Pirate Ship comes with new music. This is technically correct. If you don't own Hyrule Castle, buying Pirate Ship gives you Termina Field and the Zelda Medley.
However, this poses a problem. In the future, if a Brawl stage isn't a Mario, Kirby or Zelda stage, there really isn't any "new" music they could grab and sell as coming with the stage.
It's something that I think would actually get them to ship at least one song for a stage. Frigate Orpheon, for example, couldn't have that bullet point. However, Planet Zebes 64 and Metroid Prime Title Screen are there to easily be ripped.
If they want to continue to sell that DLC stages come with new music as a bullet point (which I feel they would), then their hand is kind of forced. Luckily, every potential stage port from Brawl has music not in Smash 4 (either from Brawl, Melee or 64), so they can easily just grab those songs.