As brake fluid becomes dirty over time, it can cause a failure of the anti-lock brake system (ABS) modulator assembly. Our technicians recommend a complete brake system flush every two years to help prevent this issue.Google+
Car Problem Reports
Mercedes-Benz S500 Flush Brake Fluid Every Two Years
Mercedes-Benz S500 Problem
I would not call this a "problem". This is good advice for any car of any make or year, though many manufacturers do not require it and few owners do it. Brake Fluid, like any fluid in the car, has the task ob absorbing and holding contaminants and moisture that will enter the system over time, from weather changes, mechanical and seal wear, etc. It needs to be changed just like motor oil, to get the bad stuff out.