Many of us use playlists to separate our music into genres, and many songs are added into multiple playlists. So, what we do is adding it to each playlist manually, which is not that bad when you have few playlist (like me), but doing this can become harder and harder over time.
So… Why not making the “Add to Playlist” feature to show us the playlists just the way it does now, but showing multiple-selector checkboxes?
That way we can see properly in which playlists the song is and also it lets us add it to a few playlists flawlessly (or at least saving a lot of time).
