I'm subscribed to a few playlists that are updated fairly frequently. Some are every day, some a few times an day. If I don't log in to spotify for a few days (ie, over the weekend since i'm a 9-5 listener) I might see 20 notifications that Playlist X has had 1 or two songs added. Why not change it so that it consolidates them all while you're logged off, and simply tells you "User has added 35 songs to playlist X" .. or something to that effect.
This idea has already been suggested here:
http://community.spotify.com/t5/Spotify-Ideas/Song
Add your kudos there please - the existing kudos here will be added soon! ;)