Only needed in motorsport really, I seam welded the chassis legs front and back on mine just for a little extra strength. TBH I wish I hadn't as it screws up the design of the car's crumple zones, which means that it the car won't be able to absob an impact as well as it should which would mean if you were involved in a head on accident you would be more likely to crease the floor pan.
I can definately appreciate the benefits if you are rallying or track racing the car as it will definatley stiffen up the shell. Seam welding alone will not be sufficent to stiffen up a shell though, you would need fillets added and gussets aswell in places.
HTH