Think of it like a crush zone in a car... it absorbs the energy, deforms, but protects the insides better than a solid, hard case.
Aluminum is a soft metal, very bendable. I used to work with it a lot on aircraft. It is quite strong for its weight, and can be turned into some very rigid structures, but it can also be bent, punched through and can oxidize in its pure form. I imagine the whole issue of material is one of cost vs. benefits, and aluminum came out the winner.
Speaking of Thinkpads - I had one for 7 years before it died. Yes, it was rugged and durable, but both sides of the case on right and left of keyboard along the edges, cracked after a couple of years of use. Also, bits of plastic would crack off in places, usually where there was some seam or edge. The main cracks in the top case near the keyboard came about by simply opening and closing the lid repeatedly. I just lived with it, but it was not classy seeing the cracks open up each time I adjusted the screen tilt. All cosmetic, but I honestly can't say the plastic in the Thinkpads is any more rugged than the aluminum in the Macbook Pros. My MBP is a year old, and I use it the same way, and it looks pretty good, and still feels tight and new. I can't speak for the unibody models... but the previous versions were a lot better constructed than most people here will admit. Especially in the later revisions MBP Classics.
