When a digital color image is mapped from raw pixels to processed pixels, the mapping is 4 pixels (R,G,B,G) -> 4 pixels (RGB,RGB,RGB,RGB). Yes, there is necessarily some loss in resolution, due to the color interpolation, but less than a factor of four. If you simply treat each raw pixel value as a luminance, rather than a color, you will get very strange color effects. For example, a bright red apple will light up only one pixel out of four, which is hardly the result you want. To really achieve better resolution for B&W, the proper solution is to remove the Bayer filter, so that every pixel sees all colors.