I see three failure modes on laptops
Hinges?! I have never seen failed hinges on a laptop. I've on 4 occasions removed a failed LCD from an old laptop, and on every occasion, more than half of the time spent was spent working over the hinges. In one case I turned to bolt-cutters. I can't imagine them being a failure point.
The failure modes that seem to be the most common in my experiences with laptops are:
Dead spinning drives, due to careless handling - SSD's render this moot
Broken power connectors
How do screens get broken? Aside from user-idiocy, generally by the laptop getting crushed while closed in a bag/etc. The metal case provides some protection against this, that you just don't get from a plastic case - especially on such a thin machine. The increased rigidity of the metal case should also help prevent the board from flexing, which should be good for reliability on that front as well.
Next time you're in a computer store, feel up a Zenbook and an Acer S3 (and the other ultrabooks if you can find them). Keep in mind that these are ultra portables, and you're supposed to carry them around with you. Which one do you think is going to last longer?
It's not like the metal case is the reason these machines cost a fortune, anyway - it's the SSD and the miniaturization required to squeeze a computer into the tiny formfactor. I've got the 13 inch zenbook, and the sturdy metal case was one of the reasons I went with it. I'm going to be carrying it around, it's going to be taking some abuse, and I want it to last. If i'm buying an expensive computer already, it makes sense to spend an extra 10% or so to get one that will last.