When I'm online, I have the option to "add to playlist" by selecting the "..." right from within the "my music" list. However, when I'm offline, this option disappears and now I must bring up the "song page" for the "add to playlist" option to appear.
Agreed! - I've had the same issue. I have cellular data turned off for Spotify to limit my data consumption, but I still often am inspired to add songs in my library to particular playlists while offline, and accessing the "Add to Playlist" option in this way requires that the song be playing, which can mess up my flow. There's no reason the "Add to Playlist" option shouldn't be available from the music library!