Plastic in automotive makes sense as does plastic in a cell phone. But plastic is plastic.
I think Apple uses aluminum because it is light weight, looks great and is recyclable. Plastic is more durable but so are plastic plates and utensils. That doesn't mean there is no reason to have fine China and silverware. iPhones fall into the latter category in terms of quality. If I planned on dropping my phone, I would probably use a ruggedized model of some sort but I don't plan on dropping it. I am very careful with it, just like I don't put the fine China in the dishwasher.
1.- Apple uses aluminum because it looks really good. Plastic is light weight, is recyclable and can look great with professional finish.
2.- Faulty analogy. Iphones v Galaxy do not compare to plastic house-ware v China and silverware when it comes to price points.
3.- Using plastic/rubber cases defeats the purpose.