I'd say most people wouldn't give a crap what it's made of as long as it works.
That's largely true, since it's basically
buy-as-cheap-as-possible-and-use without regard to how well the machines hold up long term.
The design, materials, rigidity, construction tolerances, etc., which aren't usually much of a concern with those budget, $500 machines, seem to make a difference with many Apple products.
I've got a closet full of cheap laptops from Dell, Asus, Compaq with snapped off power connectors, broken hinges, broken/faulty keyboards and trackpads.
It's not unlike the difference with the leather/seating materials used in a Corvette vs. a Porsche. A couple of yeas of entering/exiting in a C6, the bolsters are worn out, the same duration in a C4S? The seats looks brand new.
Though maybe in the end it's simply the PC market following a more "disposable" model. For the price of a MBP, you could replace your PC 3-4 times (even though I find swapping machines sort of debilitating ...)
And if plastic bothers you so much, there are PC's out there made of other things.
Well, when you start getting into the more "exotic" materials, the prices start to even out quite a bit. I've been cross shopping some high[er] end ultra-portables, and the nice metal chassis, long battery life machines (and I'm sure you know) get pretty pricey (well into MBA/MBP territory).