You can use a standard ball in conjunction with perhaps a 12mm spacer to clear your bumper in most circumstances, however the big problem here is with the zinc plating that standard balls are coated with?
The hitches friction pads will quickly rip the soft zinc off and this will cause a lot of noise while you are towing, this will then require the premature replacement of the very expensive friction pads.
The other problem is 'pitch', this is when the car is going uphill while the van is still going down, as first noted on ferry loading ramps at low tide but could be a steep dip in a road anywhere?
Under these circumstances the head of the hitch comes over and binds on the shank of the tow ball, the damage then can often require the whole hitch to be replaced, that's if the damage is noticed before the van comes adrift!
So no not at all a sales ploy!