I recently had a BMW (ZF) auto transmisstion faliure in our 2000 540iT at 140K miles. I also did proactive fluid changes every 50K miles. In this case the forward drum cracked. All the fluid changes in the world would not have prevented this.
After this experience ($5K) not sure where I stand anymore on this topic. I certainly will change the fluid before 100K miles, how much before is the question....