I posted on another thread recently that the average annual mileage for motorcycle owners is around 5,000. Dealers know the odds and in a 3 year period that 15,000 miles would work out to three or maybe four service intervals. So if the plan covered six intervals, the dealer doesn't have to worry about the last two ever being used.
The result is that cost goes away and falls to his bottom line. Plus the dealer gets to use your money up front for that three year period.
The more I think about it the less I like the idea.