Almost all BMWs, especially the SAVs, typically have notably more negative camber in the rear than other automobile makes (ultra high performance models aside). As we all know, rear negative camber is intended to create a flatter contact-patch between the outside rear tire and the road surface, reducing oversteer as the lateral forces create more positive camber. This is especially important on RWD cars, obviously, as they are prone to oversteer when power is applied too early coming out of a corner (yada yada yada blah blah blah).
Now under normal driving conditions, the X3 has a 40/60 front/rear torque bias. This is done to facilitate more spirited, RWD-like driving characteristics (but let's not get carried away...this is not a RWD car). Similar to a RWD car, then, the X3 should require more rear grip to reduce "excessive" (
) oversteer.
But because of the increased negative camber at the rear wheels, the straight-line contact patch is effectively less than than it would be if the camber angle was at 0˚ (see numerous X5 4.8is threads regarding significantly uneven rear tire wear). The car, then, has less straight-line, high speed stability. In order to counter this loss of stability, the rear wheels are fitted with wider tires to increase the width of the contact patch when quick, high speed maneuvers that won't change the camber angle nearly enough need to be made (i.e. avoiding a large pothole on the expressway at 80 mph).
This combination of a rear-oriented torque bias, negative rear camber, and wider rear tires gives the car a more balanced dynamic of fun, maneuverability and control, and safety. While the staggered setup increases understeer at the limit, the car is actually more agile at road-safe speeds. There is a line, though, and if you intend to track your "people-mover-with-an-identity-crisis" (a frugal attitude towards high performance SUVs and SAVs), the squared setup does lower rear grip ever so slightly enough to ever so slightly increase agility at high cornering speeds. Whether to actually use a staggered setup on track with, say an M3, well that's up to the technique and driving style of the driver himself.
Bren, while you're right about wider front tires decreasing understeer, this mostly applies to cars whose aerodynamic and geometric limits exist beyond the limitations of the engines by which they are propelled. BMW and insurance companies would rather argue the safety of an SUV more prone to understeer than oversteer; loss of front grip is easily remedied by slowing down, while loss of rear grip takes more skill to correct. An SUV is heavy; it takes a lot to undermine its curb weight. But when weight can't keep it all together, good luck fighting the straight-line momentum that suddenly went sideways.