I have some fairly severe paint damage (Red Multicoat) to both rear doors from driving in the snow this winter. I have 5k miles and less than half of it was in snow. That said I did install front and rear mud flaps ASAP, but by the time I had installed them the damage was done. I went ahead because at this rate the paint would be totally gone next year and it would become a corrosion problem too.
Note that I did not see any damage behind the rear tires so the rear flaps are probably unnecessary.
Another thing to note is that I have some considerable erosion to the leading edge of the rear doors where the PPF does not cover unless it is wrapped around that edge (probably should wrap the bottom edges too).
I cannot seem to be able to upload the pictures but one the rear flares the paint has been eroded through to the base white coat below over several square inches and the edge of the rear passenger door is worn through to the white also..