The whole body is part of the structure. Every piece works together to properly crumple up and protect the occupants. The only way to fix rust is to cut it out and replace it with steel, not cover it up with fiberglass.....
How's that work out on a Corvettes, Saturns, Shelbys (fiberglass dog house), Buicks plastic fenders. Not rust, but the fiberglass part? No structure?