Currently, a click on a saved playlist with the middle button does nothing. By comparison, a double click on a saved playlist expands it and offers to rename it. This offers an intuitive way to rename a saved playlist, but is not necessarily the most common action performed with a saved playlist. Double-clicking on an album adds the album's tracks to the current playlist. Finally, middle-clicking an album does the same thing as a double-click (adds the album's tracks to the current playlist). My expectation was that middle-clicking a saved playlist would add its tracks to the current playlist.
Amarok 3.0.1, apparently double-clicking a saved playlist adds it to current playlist. I think the behaviour's intuitive & sensible enough now (renaming available with F2 and right click menu), so maybe I'll just close this.