Volkswagen has the same idea with their new auto trannies, sealed for life! There is no way to check the fluid short of a computer, therefore no transmission shop can flush the system (and there are no external fitings inorder to flush the system). The owners manual specifies that the atf is good for the life of the car, not to be changed. I asked about changing at the dealer and the reply was "why would you wnat to spend $250 on oil?" (they use synthetic fluid aswell). They have no way of draining the torque converter either, meaning a change only removes 2-3 quarts (8 quart capacity). Silly what the auto makers are doing.
As far not changing the atf, this advice was founded on domestic cars, which apparently need new trnasmissions every three years anyways!!!!
Changing the fluid once it is already burnt is risky, but failure is going to happen anyways at that point, the mechanic just doesn't want the heat when it fails to frive away from the shop. Mercedes transmissions are strong and should have regular changes, I change my fluid every 50000 miles (less then two years), and at 525000 miles it still shifts and runs fine!