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.
When I lived in college dorms a few years back, my thinkpad would fall ~6ft out of my bed onto tile a few times a week. Occasionally I had to pop the battery back in.
Plastic can flex to absorb the stress without permanently deforming, aluminum not so much. Even the panasonic toughbooks are plastic, and those are probably the most durable laptops I know of by a large margin.
Interestingly enough destroying a Panasonic Toughbook is easy. Stand on it with high-heels... The Panasonic rep had just been stomping on it, and then this smallish woman asks if she could try.
It didn't boot or do much of anything after she had traipsed all over it.
That being said, I am not saying the MB would fare much better, but those Toughbooks are not as tough as they are advertised.
Several years ago while walking outdoors during the winter carrying my thinkpad in my hands, I slipped on a manhole cover that was covered in ice. The thinkpad fell about 4 to 5 feet straight onto the manhole cover, landing on it's corner (left-side palm rest corner).
The corner itself was obliterated, leaving a small hole where it used to be, but that was the only damage the laptop sustained.
Unibody metal laptops are nice, but I think the structural advantage of them is heavily oversold. Plastic laptops with metal frames can be amazingly durable.
