It should be possible to display only the background color of cover fills, without the fill pattern.
While it is efficient to show material fills directly on 3D polygons, cover fills are equally valuable for their masking ability. With a lot of display order manipulation, you can output a complicated roof plan consisting entirely of actual roofs. Cool.
If the roofs have a cover fill pattern, it's not practical to show the roofs themselves in other plans, such as site or framing plans. Too much clutter. But you can't turn the fill pattern off while keeping the masking function.