It isn't, in most cases. In the BH412, the greenhouse window above the pilot must be covered in order to fly IFR. I've had vertigo from the flickering shadow of the rotorblades on the instrument panel in a Bell 47 (TH-13), and having a window directly above the pilot can be a real problem. I almost never look through the greenhouse above me, and wouldn't mind at all if it weren't there, but it's traditional, so manufacturers keep providing it.