That's been explained before. Aluminum is not practical as it is simply not durable and can't stand up to being dropped or knocked around. I once even dropped my black MacBook from five feet up onto a hardwood floor and it was perfectly fine. Try that with yours. BTW- the last MB design was newer than the aluminums. It was great- fantastic unibody design.
Fortunately, I make it a point to not drop my laptops. I have had a white iBook, Aluminum PowerBook G4, and now my aluminum MacBook. I never dropped any of them. I guess I am just very careful that way. I have also had 2 iPhones, and even though they are smaller and much more prone to droppage, I have also never dropped either one of those. So I consider myself lucky. The only laptop I would assume is rugged enough to withstand a fall is the Panasonic Toughbook, or something similar.
Would my MacBook survive a fall? I have no idea and I'm not going to find out. Even if I knew it would survive, I still wouldn't be okay with dropping it as it would likely scratch the surfaces, and I like to keep my computers in pristine condition. If it were a junky Dell or Compaq laptop, I may not care as much, but with my Apple gear, I treat it very well.
As for the toughness of plastic/polycarbonate, my mom dropped her iBook (white polycarbonate) and it did crack the casing in two spots. She also sat it too close to a candle on her desk and it melted the top casing a bit. It wasn't touching the flame, but it was close enough to the heat. Of course this would not have happened to Aluminum. Also, a good friend of mine with a white MacBook (non-unibody) dropped it and completely ruined his hinge.
My point? Both materials are good at some things, and also have their downsides. Nothing is perfect, but I wouldn't want to drop either one. Given the choice I pick aluminum any day of the week. Likewise, it looks significantly better, feels significantly stronger, and matches Apple's aesthetics.
By the way I am not trying to convince you or anyone to agree with me. You are welcome to retain your opinions. I'm just explaining how I and others see this issue.