I don't doubt that they are space efficient ;) But for the usability part ... I'm no usability expert, but I honestly doubt that this is just me. Simply because most of the world is used to read horizontally. In fact I think a usability study might be interesting here. I'm thinking especially about new users. Having used Amarok for a few years now, I got used to it as well. But try using an app that you have never used before, which uses vertical tabs and where you're not familiar with the interface: you always have to rotate your view or even your head to be able to read the text and you can't just naturally grasp it.

Also now many things which are actually top level are hidden behind the tabs. A sidebar would allow to add more than just 4 or 5 places, e.g. serveral collections, playlist, radio stream, podcast or jukebox... This means having things more visible and in the face and the user is less needed to switch tabs. When I was new to Amarok I found myself often clicking erratically through the tabs until I found what I was looking for. Another thing is that the tabbar is the major entry point to the music sources, however visually it falls off, being so slim and at the edge.

I admit though, that replacing the tabs by a sidebar or something might mean a major redesign of the interface, for the space issues you mentioned ...

just my 2 cent :)