motrek is right.
It does not harm your batteri to keep it plugged in all the time.
The worst thing you can do to the battery in your MBA is to let it become fully discharged. Instead charge it when you can. It's designed to quickly get to 80% and then slowly charge to 100%.
As motrek mentions, Apple previously suggested letting your battery discharge completely once a month. They do not recommend that anymore. My only guess on why they would recommend this, was to calibrate the measurement of the condition of the battery.
What DO matters is the number of charging cycles you go through. There's a reason that you can view the number of cycles in System Profiler. Apple defines a charging cycle this way:
"You complete one charge cycle when you’ve used (discharged) an amount that equals 100% of your battery’s capacity — but not necessarily all from one charge. For instance, you might use 75% of your battery’s capacity one day, then recharge it fully overnight. If you use 25% the next day, you will have discharged a total of 100%, and the two days will add up to one charge cycle."
So if you discharge your laptop entirely every day and re-charge it, you will have had 365 cycles in a year. If you keep it plugged in virtually all the time, you might end up at less than 20 cycles for a year.
According to Apple:
"With lithium-ion batteries, the capacity diminishes slightly with each complete charge cycle. Apple lithium-ion batteries are designed to hold at least 80% of their original capacity for a high number of charge cycles, which varies depending on the product."
So, bottomline: Number of charging cycles matters. Keeping your laptop plugged in all the time reduces/eliminates the number of charging cycles, thus maintaining your battery in good shape.
Sources:
http://www.apple.com/batteries/
http://www.apple.com/batteries/why-lithium-ion/
Edit:
Apple actually never said to completely discharge the battery once a month, but merely to "complete a charge cycle once a month":
http://web.archive.org/web/20120917221543/http://www.apple.com/batteries/
They have removed that recommendation. But if you want to, you can probably let your MBA get down to 50% once every two weeks. That way you'll do one cycle per month - without hurting the battery.