I'm not positive but, I think the torsion spring also provides better "damping" (less bounce) even without shocks. I do recall another "mod" for the HF trailer is to remove the "overload spring" from the leaf spring pack.
you are correct .... several good, lighter weight campers use torsion bar suspensions .... they really work well .... translation, when you get to where you are going the dishes are stll where you packed them in the cabinet