Hard cases add weight that can't be a good thing in the long run. They act as thermal shields so interfere with cooling. They also trap dust particles underneath, and that's likely to scratch the laptop.
About the only thing I see them protecting against is dents.
And let's be honest, they're ugly. Why use an ugly cover to protect your MacBook from cosmetic damage ? Unless you want to sell a pristine MBA a couple years later.
I would just get a protective carry sleeve and leave the MacBook naked otherwise.
Although judging by the date of your post, I'm too late anyway.