Hi Aryan,
The granite case looks nice. I also thought about getting a skin for my MBA 13. I have one on my iphone 4, but had such a hard time placing the shield that I became bitter
It can only help though.
I take the untype sleeve everywhere and it's basically enough to stop my MBA from being scratched in my bag. My messenger bag has everything from my iphone to my apartment keys in it which necessitated some kind of protection. I have absolutely abused my mac on my trips overseas and I'm sold on apple's build quality. I was in the plane a couple of weeks ago and I put my messenger back up above me with the macbook in the sleeve. When we took off, I heard a smack/scrunch...but of course, you can't take off your seatbelt and check (and be mistaken for a terrorist or just plain dangerous) which drove me crazy until we leveled out. So I get up and check. It was fine and even though that was the case, I will NEVER do that again. I can't even count the times someone has leaned their seat backwards and almost crunched my screen/hinge. That's a plus for the 11 inchers, they don't have that problem and I'm constantly looking at ipad users with envy. The 13 inch is just an inch too big for Delta/American flights.
Long story short, the MBA can take quite a beating as long as it's closed. Zagg shield is good but I think unnecessary if it's in a sleeve.