The MacBook Air, despite what some think, cannot be a full time computer for most people. Storage space is too small and too difficult to upgrade, no optical disc drive, significantly slower processors, no upgradeable RAM. It's just not something that can be a full blown system.
You're just stuck in the old way of thinking. The 1 computer household, connected directly to the Internet. I and many others use the MBA as our sole, full blown systems and do it with ease.
External storage (why would I want to carry all my data with me at all times and risk losing it ?), good enough processor (let's face it, processors have been fast enough for most task for the last 10 years) and plenty of RAM to do what we need (not everyone is running enormous dataset crunchers on their laptops).
Let's see what kind of storage I have on my lowly MBA :
Code:
$ df -h | egrep "disk|afp"
/dev/disk0s2 113Gi 93Gi 20Gi 83% /
afp_35Bu7f35Bu7f35Bu7f35Bu7f-2.2d000004 916Gi 810Gi 106Gi 89% /Volumes/Public
Would you look at that, over 1 TB! Of course that old array is in need of an upgrade, the new NAS in the process of getting built, just lacking time for it.