Besides the clamping force hot rivets form a perfect tight fitment between the rivet shank and the holes in the material being fastened together to prevent shearing movement. A regular nut and bolt wont do the same unless the holes were reamed and fitted with close tolerance bolts that would almost be a press fit through the material to do the same thing. Huck bolts would be ideal to replace the rivets but most people don't have a huck gun in their tool box.
That's what I thought. One thing though. I don't know if it was a mistake when it was first riveted. One of the rivets, on the passenger forward rivet for the k-member, it looks like they missed the hole with the rivet on the outboard side of the frame you can see indents, on the inboard the top of the k-member is cut off, and there is a bolts in its place. ill have to get a picture of it.
