http://www.tirerack.com/winter/tech/....jsp?techid=18
Take a look at the linked article at Tirerack.com on the importance of matching tires on an AWD vehicle.
While xDrive may tolerate somewhat greater differences in tire sizes than some all wheel drive systems from other manufacturers, having a tread depth difference among tires greater than 1/8" for significant miles would pose a concern to me. I generally replace my tires when tread depth on any tire is less than 6/32" in any event to maintain handling in rain or other bad weather conditions. Having mismatched tire treads may also contribute to drivability and handling problems.
I have changed all 4 tires earlier than I planned after one tire was damaged to keep the tires matched and maintain the performance that I motivated me to purchase a BMW in the first place. YMMV.