This arrangement is highly questionable, but it does have one advantage, touched on in the last paragraph: you can write a 1bpp drawing routine that does whatever you need, and it can deal with any number of planes. Like, if you've got 4 planes, and you need to draw a line in colour 7, you just use your 1bpp line drawing routine to draw a 1 in planes 0, 1 and 2, and a 0 in plane 3.