I doubt that the average lifespan of a belt is 100,000 miles. I read one poll where over 50% of the replies were along the lines of belts needing replacing around 35-50,000 miles. And while chain and sprockets do require more maintenance and replacing, pulleys and belts are subject to wear also.
If you do your own maintenace, belt drives can be cost effective, but from what I hear dealers charge an arm and a leg for the job(from one Harley board; "The stealer wants $210 for the belt and 4 hours @ $90. Plus $100 for the tow.").
