Spotify seems to really focus on playlists in their marketing, but I don't see people around me using them that much. They just see Spotify as their iTunes catalogue, but just with a lot more music in it.
I agree completely. This is the main issue that keeps me using iTunes even though I'm a Spotify Premium subscriber.
Somebody added a ticket about this on Spotify's Get Satisfaction page: http://getsatisfaction.com/spotify/topics/more_advanced_libr... -- let's go there and vote/like! :)
That was my experience when I used it (about a year ago now). I couldn't add artists to my "collection", I could only create playlists, which led to clutter and wasn't very elegant. Personally Rdio does everything I would want.