Nitrogen has a lower coefficient of expansion during temperature changes, has no oxygen or moisture to promote the rusting of steel chords.  Airplanes use it for safety reasons - race cars use it for less pressure change when the tires get hot.  There is little benefit or reason to use it in car or motorcycle tires......you will wear you tires out or the sun and ozone will deteriorate the ouside of the tire long before it falls apart on the inside.  It is a sales promotion kind of thing to try and win a customer that might go elsewhere.  As someone previously posted - air is already 78% nitrogen and the remaining 22% has little affect on tires we use to commute.