Since you say the dealer replaced the blades, I would look for some sort deposit on the windshield that is causing the rubber insert to not slide across the windshield. I know that GM, Ford and Chrysler offer a windshield washer additive that has cleaners which loosen and dissolve foreign material on the windshield. Surely Mercedes has a similar product, although you'll probably pay a premium price through the dealership.
AMSOIL has a product
formatting link
will help maximize visibility by using the wind force to repel rain, snow and sleet while driving. If you received a completed receipt from the Mercedes dealer and want to verify that the wiper blades were replaced, you should see two different part numbers listed on the customer's copy of the receipt. If they were replaced under warranty, you should see a "N/C" or similar notation in the pricing column of the invoice (usually to the right side of the invoice).
Should you begin to doubt that the dealer actually replaced the blades, AMSOIL sells Trico Exact Fit Wiper Blades for your 2008 ML350.
Driver Side P/N T2813 ($38.75 Retail) Passenger Side P/N T2113 ($25.10 Retail)
If you'd like to become a Preferred Customer and use AMSOIL lubrication in your engine, AMSOIL has a European Car Formula:
formatting link
recommend for the extended drain intervals established by the vehicle manufacturer or extended based on oil analysis. Becoming a Preferred Customer will allow you to take a discount of 25 - 30 percent off of the suggested retail prices. Preferred Customer Program:
formatting link