I had looked into the hood vent center open center closed, years ago. I found that the 460s that had AC had the vent grill with the center of it blocked off. All 460s without AC had a vent grill that was wide open. Because the 463 comes with AC it has the closed center panel. So logically MB determined that if you don't have AC you require volumes of fresh air to pass through the cabin to keep you cool. If you have AC then you don't require as much fresh air as you would conditioned air. Additionally this grill with the closed center (as Alan mentioned) also protects the evaporator.
I think these extra vents on either end are a nice way to increase the air flow into the cabin.
With the "superior" aerodynamics of the G, I wonder how much positive pressure exists at the vent openings - and therefore how much additional airflow is generated. I suppose they wouldn't do it if it didn't help.