MBA & Shells...

I am not particularly careful with my MBA, and it really doesn't have any scratches at all. I really don't feel like it's necessary. I believe if you drop it, the damage will be the same regardless of gadget covering it or not. Personally, I have never seen a cover in person for the MBA that didn't add weight, make it thicker, and look hideous. Have you thought about a simple sleeve when it's not being used? The other idea I have is a Book Book. It is something I would use to carry the MBA in and would provide protection too. It makes it look like you're carrying a book and not a Mac. Someone did a review of it here and said the MBA fit snugly in it, but they had a problem with the zipper.

It's expensive, but I believe would be nice and worthy of the cost while providing some security.

http://twelvesouth.com/products/bookbook/

All that being said, I would doubt there's anything blocking the vents on the MBA case that you have identified. Surely there are vent cutouts in the case you have in mind?