That is pretty lame. For a modern PSD that can work well on B20, it isn't a deal, but in theory, it would cost big bucks if one fills up a Sprinter on this, since it would void the warranty on the vehicle.
Well, not technically. The Magnuson Moss act means that filling up on B20 won't -directly- void the warranty... but if the DPF is fouled, the computer is throwing codes left and right, and the vehicle spends more time in LHM mode than out of it, it isn't Mercedes's fault.
Same with the ProMaster. Its diesel engine is rated for B7. Getting a surprise by B20 can mean hefty repair bills if it decides to throw a code.