Actually no its not. Aluminum is stronger and more durable than plastic. Is it more prone to chipping? Maybe, but these chips don't compromise the structural integrity of the phone. I have never seen a car made out of polycarbonate but I did see a car fully made out of aluminum.
And I don't even use cases so don't get me there. But if you must. Most cases (not all!) are made out of plastic for economic reasons. If you think they offer any protection then you are wrong. They might possibly protect the phone against scratches but that is sometimes also a myth as the case itself might scratch the phone.
Totally agree with this. You can find cases made out of different types of metal, but they are typically going to cost more than a plastic one.
When I had my 3GS, the plastic on the back towards the top was cracking. It cracked on my brothers too. Also around the dock at the bottom. A lot of people had their plastic start to crack. You won't get cracks with aluminum. You can get scratches, just like you can with plastic. And in some cases dents. But if you do something to dent aluminum, that same force would likely have done a lot worse to plastic.
My dad dropped a metal book holster thing onto his MBP. It was a pretty big drop too. It made a dent on the top of the casing. It didn't affect the screen, didn't crack open the casing, or anything like that. Just an indention. If he didn't have a MBP and instead had just some generic laptop made with generic plastic, it would have done a lot worse.