It's simple to implement, yet very effective idea. The ability to play from several playlists at the same time would provide users a lot more flexibility.
Say your playlists are organized by genre AND by decade, (ex. hip hop 1990s, hip hop 2000s, R&B 1990s, R&B 2000s) you could want to listen to all decades of a specific genre and select both hip hop 1990's and hip hop 2000s, or if you want to listen from a decade you select hip hop 1990s and R&B 1990s.
Understanding you can currently add different playlists to a queue, but the shuffle doesn't work very well when using the queue and there's a limit of 50 tracks.
Understanding you can manually merge two playlists together into a bigger playlist, but that becomes unwieldy as there are many combinations that one could want and when you add a song to one of the "building blocks" you would also have to add that song to any other "combined playlist" you have, which is a very manual process.