a 7.5 is rated at 2700lbs. a single 6-ply tire is rated at around 2,000lbs. times that by 2, a pair of 6-ply tires can carry 4,000+lbs...over a thousand pounds more than your axle is rated to carry. add to this the fact that those wheel adapters increase the payloads leverage on your wheel bearing and axle shafts (thus DECREASING your axles usable payload) and its easy to see that you arent accomplishing anything by adding dual tires to your stock axle.
you may have driven 20,000 miles with the adapters installed, im not saying you didnt. but you didnt drive 20,000 miles at or near maximum payload with those adapters installed or else the axle WOULD have failed.
the kits are 100% aesthetic and shouldnt be confused as anything but.