'There has been a few instances of a roundy round hornet flipping on its side, drivers side down, and being T boned. Never ends well'
I started out in racing helping a friend build and race a dirt track car, that is where I noticed the soft underbelly of these beasts. The floor pan isn't much thicker than the door skin, imagine getting hit in the door if there were no bars there.

As far as a rule, how about just saying one or two optional bars between the rear hoop and the firewall on the drivers side of the tunnel would be allowed to mount a seat or for driver protection. That is how we did it on the F Prod car I work on now, just ran a couple pieces of tubing from the bottom door bar to the tunnel.