I realise that i'm probably in a vanishing minority, but give me a few mm's of extra thickness for replaceability of parts any day! The 13" 2009 Macbook i refer to is (for me) fine as far as weight and thickness go, while providing replaceable parts which makes me happy (and soothes my reuse-repair-recycle tendency).

I've got a Lenovo T450S, which is apparently marketed as an ultrabook. It's got a user-removable battery. I assume you had some different ultrabook in mind with your comment.

Being supernaturally thin is not an engineering requirement. Marketing dictates that.

To be fair, I thought that its was all crap when I first saw the MacBook air. Then I acquired one through work for a couple of years. Its was nice going somewhere with hand luggage and being able to fit it in the top of my bag relatively easy compared to my other laptop (though the thinness didn't make a huge amount of difference).