its the quality that counts and not the extra weight reduction.Maybe for you. But not necessarily for everybody. I personally would also prefer a Mg body, but neither my 350D nor the 400D body broke and they're both plastic.
Besides, I don't know if plastic is really (that much) lighter than Mg. Mg is already very lightweight although it's a metal. I think the difference between the weight of a xxxD and a xxD comes also from the size of the body.