I have the Billy Goat SV50H lawn vac (older model). What everyone has said thus far is accurate, but I don't think it's been given the proper credit. What if the back yard is fenced in and all you can fit is the Billy Goat or a 21" mower?
I have had requests to suck up a massive amount of acorns before and it worked awesome. The lawn vac does that final touch to get what all other methods missed. Customers also appreciate your ability as well to vac the curb / street.