Why would you need an empty workspace (except at the end, 
  which gnome provides)? Seems to make since to me. 
  It doesn't really "re-arrange apps" it just moves 
  the work spaces to fill in the empty gap.

To each their own, I guess, but I think it's related to how we mentally map our workspace. For example, I know that my code editor and shell are on workspace 2, and my time tracker and music player are on workspace 4, so it's two keystrokes to get between them. If workspace 3 were munched due to being empty (I normally use it for Github and code review), it would be __very__ jarring to be in the wrong workspace when I switched workspaces twice to the left from my last workspace.

In practice, that's not likely to happen to me often, since I leave all of my workspaces open and populated nearly all the time, but it's still an option that I would turn off if I could.