JCL hit it pretty good
And in my non BMW experience from previous shops was that the clutch material in the fluid added some viscosity and changed the friction profile of the fluid, so when people changed all the fluid of an older, higher mileage car the transmission would inevitably start slipping. (I'm sure not being able to reset the trans adaptations with the fluid change didn't help)
I've personally seen more transmission failures on transmissions that have been serviced than one ones left alone. And since I got the vehicle with over 80k miles, I couldn't see any good coming of a fluid change. Now with over 90k miles it still drives perfectly and I'm waiting to see how long it will go before I have to go all swissfrank on her ass.