I'm trying, from a physics point of view, to follow this strange discussion.
Do people who buy BMWs not have enough funds to get both xDrive AND snow tires? And I'd think xDrive and snow tires both help handling is slippery conditions, either alone or doubly together. Why the extended discussion of either/or? I bought the xDrive, and if I need snow tires, I'll get them too.
And I don't get the claim that xDrive only helps while traveling in a straight line and accelerating. Whenever a wheel starts to lose grip, its traction diminishes, dynamic friction is less than static friction. Unless one is just coasting, with no breaking nor accelerating on any of the wheels, xDrive can reapportion the force on all wheels to reduce the number of wheels slipping and the differential rate of slip. Wheels can slip (or have traction) accelerating, breaking, or from centripetal force while turning.
The statements "A car using only 50% of its capability to turn feels very different than a car using 85% of its ability in the exact same turn" and "a sports car feels much more comfortable than a sedan in a corner; it is working much less hard" are also mysterious to me.