On the one of the two Jeep Cherokees I had, the optional 'air deflector' at the rear was designed and sold to split off a thin curtain of air that was going over the roof and re-direct it to go down the rear window.
This actually did work to prevent a lot of the dirt/dust/muck sucked up from the road and under the car from swirling into the vacuum behind the vehicle and plating itself onto the window.
In direct comparison between those two rectangular vehicles, the one without the deflector ALWAYS had a dirty rear window, the one with did not.
The optional rear 'spoiler' for the xB does not appear designed for this effect at all, but more to give a clean break to the flow along the surface to reduce the vacuum behind the vehicle and therefore to reduce aerodymanic drag (by providing a sharp break in the trailing edge, and a slightly 'concave' rear shape so the the airflow has to break cleanly rather then trying to follow the surface - basically a Kamm tail).
All in all, though, the xB spoiler seems more 'fashion' than 'function.'
(The deflector on the Jeep didn't even pretend to be 'pretty' or 'aerodynamic' - it had one function, and that was to keep the window clear at the cost of some additional drag.)