I also considered some of the after sale options including alloy care. (Another thread).
General consensus was that it wasn't worth the cost with the following reasons provided:
1. superficial marks - man in a van
2. significant marks - has to be sent away. Not sure what MB do to keep you on road in meantime.
3. if you're considering selling car within a few years you could get all 4 alloys brought back to new before you get rid of car for less than option.
Most wheel insurance has clauses for Diamond Cut, normally they will only repair them once or replace the wheel upto a value of circa £200, which is nowhere near the cost to replace.