Yes that would be an advantage over a more traditional square rear end. That plan taper (narrowing of width of vehicle) helps to reduce the rear wake of the vehicle and works basically like a kammback, but for the sides. I would not even be able to venture a ballpark guess on what sort of improvement they are getting by doing that, but I would bet it certainly doesn't hurt! You will see similar shaping at the back of almost every aero focused car, less wake + decreased turbulence will always be good.
EDIT: Metro can type faster than me.
