Interesting! The 'stagnant air' on the rear glass and trunk is what you'd call the "boundary layer" of air. Its a natural thin layer of air that clings to any object moving through air. It actually is not necessarily a bad thing. The wing looks cool with all the strings on it showing the air stream moving off of the car.
But back to this boundary layer:
There's a reason why golf balls are dimpled (go with me on this, it'll make sense in a minute). Remember that boundary layer of air? OK, objects moving through air creates drag on the object, so you can't go as fast, or you spend more energy cutting through the air. Well, putting dimples on a golf ball makes that boundary layer 'stick' to the ball. Why would you want that? Because air slides against air pretty easily (as opposed to an object moving through air). So a golf ball glides easier and straighter through air with the dimples since its 'coated' in a layer of air. If we had smooth golf balls, they'd be all over the damn place and not travel properly, fall short, etc.
Back to our beloved Nismo:
Maintaining that boundary layer of air may (in whatever small amount) help reduce drag on the car. What you don't want are sharp bends, pockets, or anything else that isn't aerodynamic and would cause "vortexing" (completely disrupted airflow). Vortexing is bad, mkay? That causes big time drag. Which is a drag. (sorry couldn't help myself)
If this has left you feeling
I understand and I'm sorry. I believe I've confused myself as well.
If you don't believe me, try it yourself!
-Open car window
-Drive and maintain speed. Do it on the highway.
-Put just a finger or hand outside but press it FLAT against your car door. Be one with the car.
-Notice how you dont feel a heck of alot of air whipping past it (you're IN THE BOUNDARY LAYER BUM BUM BUUUUUM)
-Ok now lift a finger or two. Notice how much you feel the air whipping past it (OUT of the boundary layer BUM BUM BUUUUM)
Ok that is all. And I'm sorry.