sorry for not explaining myself correctly...you are right...it is for stiffness...by decreasing the distances between each weld (whether spot weld or not) you increase chassis rigidity. This can also be achieved using a cage, but it is bloody hard to build a cage around the front end. Thus seam welding is usually done around the front of the car, in and around wheeltubs, while the primary source of stiffness is a cage. You cant go past good seam welding, as a cage by itself or even stiff suspension can still tear seams apart.