I think as you say in your last sentance, the main reason we would use these is for the reversing factor. I'm not sure I would have it onb while driving in a forward direction to see what's behind me.
However I might have second and third ones at a later date (just because I can sort of thing).
to combat the headlight and sunlight glare I think the cameras would have to have some sort of automatic iris which would reduce the amount of light coming in.
Maybe someone needs to butcher some nice expensive digital video camera which has this sort of function and try that. (any volunteers? )