and yes I too keep my wheels clean. My 12 year old Jeep with painted wheels gets much more abuse, less cleaning, gets driven in the snow, and does not have the problem. As the original poster I appreciate all the advice on how to attempt to avoid the problem, but I do not expect to have to clean the wheels twice weekly to have them function and stay looking good.
It would be like a dinner plate manufacturer saying....sorry the tomato sauce stained your plates, they were not designed for that!!! Did all the manufacturers (Mercedes in this case) not know brake dust would get on the wheels? Jeep figured it out. The fixes given here are like telling me to put tin foil on my plates before I use them....or don't eat on them longer than 15 minutes or they might stain. We would not take that from a plate maker...and should not from a car manufacturer.
And the dealer tried to get the pitting, material, whatever....off themselves and could not either.