The topbar in Unity contains the menu for the currently focused window (and, in the future, hopefully the currently focused application). This vertical space had to be used anyways - the clock and some sort of notification area are essentially mandatory.
As a person who primarily uses Mac OS X, I think that the menus are the best use for the vertical space that those items provide. I can use Cmd+tab or Exposé for switching applications, so a list of the currently open windows isn't particularly relevant. By moving the menubar to a single spot, if anything we save space.
That sidebar is truly awful though.