some of the design benefits:

do you ever take off your oil cam and their is a milky film on it? that is caused by condensation that sits on the inside of the plastic valve cover because the plastic valve cover stays way cooler than the rest of the engine, the metal valve cover will heat up to the same temp as the rest of the engine and help burn away most if not all of that junk, 2.0 FSI's have it real bad and so did the old 2.0 N/A motors when they changed to plastic valve covers..junk that doesn't need to be inside the motor deserves to be delt with. the benefits are for the sake of showing this isn't purely cosmetic