When Tim at SBC was installing my window brackets, I recall he commented that every VX he'd ever seen had only 1/2" or less of space between the rear bumpstop and the axle. He said mostother trucks he's seen had several inches of clearance. Mine appeared to be sitting on the axle. This pic was before I upsized my tires.
I, too, can't see how they could smooth out the ride, or contribute to vibrations like Eric mentioned.....I guess if they smoothed out vibrations, they'd have to be sitting on the axle....which they do on many VXs. So I guess it ends up being a choice for us of: less vibration OR less slamming/jarring
As FYI -Here's one of the front bump stop (I believe there's two sets up front).