Looking on the dark side as Ash suggests,that plate is where your feet are should you be unlucky enough to get hit midships and it would go a long way in stopping the chassis deforming.
I am thinking of a full plate just because it can be cut in one piece easily and folded to fit,it also forms a firewall and possibly a little know fact about accidents is the cause of fires is attributed to brake fluid being spillt on hot surfaces and anyone who has done any heating or welding near a flexy pipe will understand exactly what I mean.
