There is no reason why Spotify shouldn't have a functionality on at least the desktop app where you can drag one song into as many "selected" playlists as you want and have that song added to all of them at the same time…
Here's a little more info to back up this request:
I'm, for one, the kind of person who obsesses over music, and a huge component of that is the way in which I organize my playlists.
I believe it is probably safe to say that most people wish their music was better organized, but the primary reason why they aren't is simply the amount of time it takes to put together quality playlists. Instead, what most people end up with are a small handful of enormous playlist hundreds, if not thousands, of songs that have no rhyme or reason to why they are in the same playlists.
To avoid this dilemma, I have spent hours and hours sorting through all my favorite songs and making different types of playlists that they may fit into. One thing I learned early on is that a particular song is likely going to fit into multiple playlists, and this is where the problem lies and the primary reason why most people avoid making well thought out playlists all together.
In order to add a song to all the playlists I want, I have to sit at my computer and continuously drag a new song I want put into different playlists into each individual one over and over again which, when organizing dozens or even hundreds of songs, can take an insane amount of unnecessary time. And that is if you happen to be creating and organizing playlists on the desktop app… if you are trying to do this on your phone, you might as well forget about it - it would take you an hour to add 20 different songs to 10 different playlists.
That is why I believe Spotify should add a function (starting on desktop and eventually moving onto mobile like they are likely to do with the "folders" for playlists) in which a single song can be added to multiple playlists at the same time.
Attached is a picture originally posted by a subscriber named anaraee who had requested this a few years ago, but apparently wasn't able to get enough traction them... hopefully this time will be different.