I think it would be very useful to include other playlists in a playlist. The use case would be in the event that I have two (or more) playlists, perhaps a alternative and a pop playlist, then I could create higher level playlists that simply shuffle several playlists. When any of the referenced playlists were altered, these higher level ones would automatically be updated too.
Updated on 2020-03-02
Playlist folders are already available in Spotify. In the Desktop app, right click in the playlists area, and you'll find the option to create a playlist folder. Once created, you can drag and drop playlists to the folder for better management.
Hope it helps. 🙂