I actually dig the patinated look of my dented and scratched old MBP and my current MBA. My plastic Dells from work always look terrible after a few trips living in my backpack and getting banged around. Haven't had actual usability issues with either though.
Plastic is great, until your laptop falls and the plastic shell shatters. That's the weakness of plastic - it's brittle. I have a ten year old macbook with a dinged aluminium chassis. The structure of the shell is still intact despite a few falls.