Well it is dangerous to venture into armchair aerodynamics but I'm gonna give it a shot anyway: I think when we "break" a rear slope then re-attach thus trapping a vortice, the vortice really needs to be trapped so it sits there "rolling". If there is flow through that region I'm guessing it won't allow a trapped vortice to form. I think if flow comes in through the sides and goes out the back that yes, the flow down the back could be nice. The "but" is the same thing I've said about ducts: now you have internal and external flow drag.
And then there is the ever-present cross-wind flow...
If I EVER get around to finishing my aerolid
I am going to incorporate a rear window treatment as the Chrysler PNGV car has.