I just tossed my Macbook Pro across the room the other day by accident (didn't check that my backpack was closed...). It did indeed bend the metal a bit, and the display cracked in the corner, but having seen plastic machines make the same tumble I think it would have fared significantly worse if it weren't aluminum. The force that bent the metal would have surely cracked the plastic into pieces and the display would have taken more of the shock and cracked way worse than it did.