I also have used these copper test strips to verify the brake fluid -
Amazon.com: Phoenix Systems (3003-B) Brake Fluid Test Strips, 2 Strips Per Package, Reveals Brake Fluid Condition: Automotive
Unless you are tracking the car or really in some excessively humid areas, the brake fluid takes a while to really become contaminated.
I guess BMW wants the brake fluid to be a fresh as possible.
2 years is a smart idea, but may be excessive as well.
Lifetime transmission fluid, but every 2 years on brake fluid! Go figure. On is a safety system, but only a small percentage of cars on the road ever get brake fluid flushed on a regular basis. Some never get the fluid flushed.