Most of the commercial ones I've looked at have a much heftier bottom pivot putting most of the strain onto the rear crossmember, only really using the tub capping mounted pivot as a locator.
Maybe with a station wagon or van roof it wouldn't be too noticeable (the rigidity of the roof and door would help) but I'm not sure I'd trust it that much on my truck cab. If fixed closed it'd be supported both sides of the door/tailgate, but when it's open I could see it peeling the rear panel off my tub...
Most of the weight is transferred to the bottom pivot, its basically a big triangle. I have hung on it in the closed position (17.5 St) and it didn't budge. The door frame strengthens the top mount.
