I think the reason why people like the "power flushing" method of changing transmission fluid, is that the machine guarantees to change every drop of fluid.
In most transmissions, more than half of the fluid remains in the torque converter, and is not drained when the fluid is changed by dropping the pan or pulling the drain plug.
Mercedes is the only transmission that I'm aware of that allows fluid to be drained from the torque converter, which has its own drain plug.
Thus, the benefit of using the power flushing machines is really nullified.
2001 E430, Bourdeaux Red, Oyster interior.
1973 280SE 4.5, 170,000 miles. 568 Signal Red, Black MB Tex. "The Red Baron".