In some cars the whole armrest can actually be reversed (turned over) thus providing a new surface to view and rest on. (Or the cover can be removed and put back on with the former lower surface up) If that is the solution, I would put some kind of protectant leather treatment (like Meguiars Leather Wipes) on the newly exposed surface right away to avoid unusual wear. Of course, I agree that Mercedes should replace if it is a defective item that has a history of such discoloration.
Otherwise, it could be that the person resting their arm on it is transferring a chemical from their skin onto the armrest that causes such a problem. It could be a lotion or something like that?? Knowing Mercedes parts prices, that is probably a $100 plus item to replace or even a lot more.

__________________
Ben Harrison Carter
1999 Mercedes ML320 87K
1992 Mercedes 300CE 89K
1995 Corvette 29K -- Sold Dec 09
1989 Mercedes 420SEL 99K -- (Sold 4/08)
1968 Mercedes 230S (106K) (Sold 9/06))
1976 Mercedes 450SEL 130K (Just sold - 06)
1961 Mercedes 220Sb (sold years ago)