This is by far the best feature of the Spotify iOS app when compared to the Rdio app: being able to take action without needing to be online.
In the Rdio app you can sync music to listen to offline, but since you’re offline you can’t take any action on the music. If you want to add something to a playlist you have to remember to do it later. The way I’ve been doing that is by taking a screenshot of every song I want to remember. There were some days I’d sync my iPhone and have around 20 screenshots to go through.
But the Spotify iOS app will let you take action on your music, even when offline. It’s more like Instapaper, which will let you like articles without a connection. All the actions will go through once you have a connection.