I would argue the need. If the cage is fit tight to the unibody, you can plate the inner rocker panel and the tunnel. Run tubes between the rocker and tunnel, in front and behind the seat. Then build your seat mounts off of that. I have looked at lots of wrecked cars and have never seen any large movement in the drivers floor pan. Some small tears on a Shine customer car that hit NHMS oval wall and then got rearended. That was a massive hit and the seat did not move, even though there was a little tear in the floor underneath. Beran Peters also front end crashed his AS car at Mid O so hard it ripped the trans tunnel. I do not believe the seat moved on that one either. For safety sake I think we should be able to do anything we want including 360 welding to the unibody. It would also make cars live longer by stressing the unibody less. But I think the seat rule, if changed, will just open up a can off worms and add little true safety. Just my opinion.
Chris