Also, not only is it essential protection for a tbone, but it's also part of the overall structural rigidity of the chassis in a frontal or rear end collision. The metal doors act as a truss from front to back of the car aiding in the pillar and bottom chassis for rigidity in its longitudinal direction. Make that out of cf with no rigidity in that area and it becomes a new crumple zone (which seeing as you're sitting there, you don't want it to be).