Unless BMW has some magic diffs / trannies, I don't believe the lifetime fluid story. All metal gears / moving parts grind a bit, release metal shavings contaminating the oils / fluids.

How quickly it 'goes bad' I think is up to driving conditions, style and dumb luck. One of my favorite indy mechanics told me to change the fluids every 60-80k miles depending on how anal I was and my budget.