From
the BMW page:
under normal conditions, drive forces are spread out with a rear-wheel bias. But the moment the system senses excessive wheel slip, it subtly shifts torque to meet the changing conditions, offering maximum available traction and superior handling on all surfaces.
In this recurring argument, on one hand, pits people who track their cars and find their rear-drive wheels NEVER slip except in winter on ice, so save some weight and skip x-drive.
Or the other side is BMW itself, which claims "superior handling on all surfaces", and the rest of us who like the idea of having four wheels pulling instead of two.
Who are you going to believe?
As an aside, it seems the 1-2 mpg are critical to save in this case, but twice the savings do not warrant suffering through the automatic start-stop bumps nor getting the 328 rather than 335 motors.