Here's my 2 cents, coming from the perspective of ownership of the very first laptop- the GRiD in Mid 1982 (paid for by my company on Wall Street, not out of my pocket!) through the very first Apple laptop- the PowerBook 100, all through the years and various lines, 540c, 3400c, TiBook 677 DVI, and now to 2 of the MBA's- two of the first Rev A's and now a brand new Rev "E" 2011, 13" i5, 1.7Ghz 128SSD/Sammy.
Even with the surest hands you will drop it, or somebody will mess with your laptop, and to my experience there in no case for a Mac laptop that will protect it from a long drop onto a hard surface. Period. Although I have heard of a polycarbonate/padded vault- which has way too much open space inside for the MBA in my research. So the best you can expect is some sort of case to offer some protection against various assaults. This breaks down into two areas I have found.
Working cases, those that stay on and you work from, and sleeves for protection in a more protective carrying cases.
There was a case that snapped on my first MBA, 2 parts, upper and lower, fit perfectly too, lightly-padded, and leather-covered, but with polycarbonate underneath. Great padded feel, added a pound or so and some thickness and was great for all scrapes and many minor impacts, but it sort of defeated the purpose of an ultraportable premium laptop. They look great nekkid IMO. So that eventually ruled out this great case- whose name escapes me now. I just can't imagine a keep-on case that I would always want to keep on and work from.
Then there are the brittle keep-on covers- but since I don't like cracks, and even the normal every day bumps IN THE CARRYING CASE will crack most if not all eventually. And none of them are worth looking at all of the time. I don't want a pink or shiny plastic-looking laptop. Sorry.
So this to me leaves either skins- of which I haven't found any of that I want to look at more than my bare MBA- although there are some leather skins which seem a nice compromise in the scratch-prevention area- which I believe is one of two other areas to focus on.
Unless you can guarantee that nobody else will handle your MBA!
I have had three broken in my ownership. Once a child in a Starbucks poured a latte onto my keyboard while the parents laughed. Don't ask. Insurance covered this one. Once somebody caught the headphone cord and pulled the laptop onto carpet from a table. MagSafe could not prevent this one- unless there was something similar for the audio jacks! The person manned-up and paid for the repairs out of their pocket. (Dead HDD, bent corner & cracked screen.) Second time it was kicked, another person paid for repairs. (Killed HDD, audio /USB board damaged) Bottom line if you have sure hands, use them to keep your MBA out of everybody's hands, and it should be safe from damage during usage. But I would NEVER put the MBA into any kind of a case without some kind of a protective sleeve. This is the only thing I have found that works for me. I use an incase, and I know some people hate neoprene, but I don't because I only use it to provide SOME protection until I can get it into the actual safe carrying case- which for me is then properly secured in a Timbuk2 or Waterfield bag or aluminum briefcase- all of which are easily made safe with the right sleeve. I have heard there is one sleeve now with memory foam- which might be better than neoprene, which I would love to try out.
To sum up- nothing is really needed during usage if you have control (and good hands), except maybe for a slim skin. IMO a sleeve is a necessity for scratch-resistance, some padding and minor impact resistance during casual transport. And then for additional padding while INSIDE your final method of carry.
I have been experimenting with this system for literally 25 years, through almost 20 different Apple/Mac laptops and for me I have found nothing better. Now I take my MBA everywhere I go, to filthy job sites, on the subway in NYC, on planes and in cars & trucks everywhere and it has never failed me yet. Padded sleeve and a secure carrying case/briefcase or bag. YMMV.
Skin or film- if desired for scratch resistance during usage, or appearance.
Padded slip case- required IMO
Carrying case with method to secure the laptop in sleeve inside, and to prevent weights from crushing, or damage from dropping.
