The trouble with the M-Mobility kit (I also had with my '03 M5) is that it expires after some time (2-4 yr?) and needs to be replaced, even if not used. Even if that were not true (e.g. some drugs can work well after expiration date), would you want to chance it when you need to depend on it? Also, although I never did get to use it, I heard it can really mess up the inside of the wheel, especially one with TPMS sensors. I also have AAA and the BMW Roadside Assistance (as you probably do as well). However... you might want to check out this thread if you have not already, w.r.t. towing/flatbedding the X3 (not related to a problem with a flat but just to be aware in case you find yourself without power/ignition):
http://x3.xbimmers.com/forums/showthread.php?t=649204
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2013 x35i (Space Gray, MSport, CW/CWII, DHP, Premium, Premium Sound, Technology, BMW-Apps)
Retired: '09 C300 4Matic, '09 GTI, '06 750i, '05 C55, '04 R32, '03 M5, '02 GTI, '01 M5, '00 M-Coupe, '00 540i/6, '99 M3 Coupe, '98 M3 Coupe, '96 328i,... '86 4000 CS Quattro... too many to list.