Yes they are cheaper to make. Because theres no sense in making non-standard screens that only a small number of people like, while everyone else would rather have 16x9.
Considering the industry, except Apple, moved away from 16x10 virtually overnight, I'd say everyone wants 16x9.
I have a 1920x1080 display right now. No 1920x1200 please. I don't want the black bars on my movies to look like they used to on CRT TVs.
Have you taken apart an aluminum unibody system? The bottom casing IS hair thin. It's about as thin as the aluminum face on the iPod classic. The casing on my iPad is thicker than the bottom pieces of my 13.3" aluminum MacBook.
I had two original plastic MacBooks (one I bought, second was a replacement), and that replacement system was replaced with the original aluminum unibody MacBook. Why two replacements? Poor case design on Apple's part. The aluminum unibody design is leaps and bounds better than their previous designs, but its still years behind what PC manufacturers can offer.
Well, then you haven't looked. Mobile Core 2 Quad showed up a long time ago. In 2008 actually
http://en.wikipedia.org/wiki/List_of_Intel_Core_2_microprocessors#Quad-Core_Notebook_processors
True quad core mobile Core i7 have been around for about 6 months now
http://en.wikipedia.org/wiki/List_of_Intel_Core_i7_microprocessors#Quad-Core_Mobile_processors
Heres a nice little Core i7 quad system
http://www.newegg.com/Product/Product.aspx?Item=N82E16834114803 $899. $1300 less than the dual core Core i7 MBP.
Also, all of those quad core Core i7 mobile processors offer hyper threading as well, so you have 8 logical cores.