1. German cars, not just M-B, have service recommendations which assume they will be driven under autobahn conditions, which means the manufacturers have to be concerned about the effects on brake fluid of repeated stops from very high speeds. Extreme hot and cold cycles do increase the hygroscopic rate of the fluid. Mountain drivers, take note.

2. Water absorption in humid climates, obviously, is a bigger issue than it would be in the New Mexico desert or in a very cool but relatively dry climate. We never had big rust problems with old brake fluid when I was a kid in Minnesota, but here in Tidewater Virginia, I would definitely follow the every two year recommendation, as I would if I were forced to live in Florida.

Therefore, you can apply a certain amount of critical judgement to the brake fluid change interval, but you'd better be sure that you really know what you are doing. It is also true, as someone else suggested, that old Pontiacs' service schedules assumed that the brakes would wear out often enough that the fluid would be changed as part of wheel cylinder rebuilds every couple of years. For those who just threw on a new set of shoes without even looking at the cylinders, this could be disastrous.