Like I posted earlier, I agree with Keith, but again, we're talking about two different types of forces acting on the frame. He's talking about vertical loading- heavy weight pushing straight down on the frame, and strengthening a frame to resist downward pressure from high towing weights. I'm trying to reduce the twisting motion of the frame, not keep the frame from bending downward when I've got the bed loaded down.
This pic represents a frame rail under torsional strain. It's twisted down it's length, much like a torsion bar would be under a load. Much different type of load applied to the frame rails than vertical loading.
This is a vertical load bending a frame rail (left side of pic, ignore the right side). It bends downward. I'm not trying to prevent this from happening because I'm not planning on towing a gajillion pounds of weight with the truck.
Now look at this picture, notice that the front crossmember is level. Which of the two pics above does this match? Only one of the rear bags are pumped up. This is a torsional load being applied to the frame, and the frame is absorbing the load instead of resisting it. If it was resisting the load the frame would be straight- the front crossmember would be sitting at the same angle as the rear of the frame, and the front suspension would be absorbing the load instead.
