This feature will be back in an iOS update coming early 2013.
This iOS update will also include the new follow features that Spotify just announced.
This might be a full redesigned upgrade... (saw some teases on Spotify's YouTube channel)
View this thread to see why the feature has been temporarily removed:
"We decided to remove the feature temporarily so we didn't have to delay releasing 0.5.9, because there were a lot of crashes while editing a playlist, tracks not ending up where they should etc. We will introduce the feature again in an upcoming release as soon as possible."