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.
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.