Originally Posted by HipToBeSquare
Unless the front wheels spin, there are no extra losses, that is the point of faux-wheel-drive systems that are normally 100% FWD (or 100% RWD with front-wheel assist, but better inherent dynamics)
When the rear drivetrain is inactive, when the car is stationary or the rear wheels are rolling the same speed as the front wheels are pulling, the only drawback is vehicle weight. The system is designed for negligible drag losses in normal operation, to preserve MPGs. The tradeoff is that it is in no way pro-active in maintaining traction, and only activates after front traction is already lost.
The rear wheels are not normally engaged, and only engage when viscous fluid heats up due to speed differential between the front transaxle an the rear differential either being still, or playing come-along.
You're completely ignoring rotational mechanical losses of the driveshaft, PTU clutchpack, rear axles and rear diff.. all of which are absent on the 2WD models. They constitute a parasitic loss in addition to their weight. The only time a faux-AWD system is more efficient is when the car is travelling on the highway and there are different tires or tire pressures resulting in different front and rear axle speeds. In a Subaru with Continuous AWD, VTD or DCCD sees losses in those situations thanks to mechanical drag on the center diff because it has to spin the spider gears in a gear oil bath (thus wasting energy). Active AWD is a "faux-wheel-drive system" by your standards. An 4WD truck doesn't get the fuel mileage of a 2WD truck just because you haven't engaged the transfer case... you're still spinning all of that extra drivetrain.. but you're dragging it down the road instead of driving it directly.
Like the new Forester (Active AWD), the MINI system uses the car's VDC sensors to maintain an aggressive lock-up curve before slip should occur... and unlike Active AWD, the MINI can lock up individual rear axles from the rear diff (for some torque vectoring)... so while, yes.. it's still a faux-AWD, it's not necessarily crap.