That lower brace possibly helps with chassis stiffening, but as it sits well below the bumper height of just about every vehicle you'll be likely to encounter on public roads, it won't help your chances much if you get t-boned at an intersection. Cobras and safety don't fit well into the same sentence - it's the nature of the beast. 4-wheeled motorcycle is a good analogy. Neither will fare well in any type of accident, and both will burn your leg with hot exhaust pipes. Gotta love them though - just enjoy accordingly.
EDIT: RE: the original question; I've always been amused by the big deal we make about roll bars on our Cobras. Few or no other convertibles/roadsters had them until relatively recently and the only original Cobras that had them were intended to protect a
HELMETED racing driver on the track. They were not deemed necessary on street driven cars, and in fact pose a very real threat to the noggins of un-helmeted drivers and passengers who are belted in upright with 5-point race style harnesses.