Her's an idea, merc wanted a sporty look to their car so they descided to put some 'go faster' vents at the back. They had a choice, put the vents into the metal rear quater panel which is a single piece along with the floor, pillars and roof, cut a hole out of the rear quater panel and put in a plasic vent or cut the vent into the perspex light cover? The last option proved to be the cheapest and easiest to do so they descided on that. Its nothing to do with aero, its all marketing.
Murphy's 9th Law of Technology:
Tell a man there are 300 million stars in the universe and he'll believe you. Tell him a bench has wet paint on it and he'll have to touch to be sure.