It will probably be fine without a sleeve or case
If you are careful with it. For example, my head is pretty fragile but I walk around without a helmet. So if you are going to treat the MBA as well as your head, then I don't think a case is necessary.
I do however wear a helmet when I ride a bike because of increase risks that I take when I do so. So ask yourself if having a MBA in your backpack filled with other hard and heavy stuff like books which occasionally get thrown around a bit is like your head without a helmet on a bike.
Well, bad analogy. It's just a laptop which you can replace, unlike your head which you cannot. So forget it. You don't need a case for the MBA.
However, if you want a case to protect the MBA, and you don't want to spend $40 on a Incase neoprene, then I would suggest the Be-ez la robe which you can get from Amazon for about $23. It's made out of memory foam, looks a lot nicer than neoprene IMHO, and is probably several times more protective than neoprene. My thought is that if you are going through the trouble of getting a case and using it, it may as well be really good as protecting the MBA.