Some people mustn't have enough real problems to worry about. Worrying about keyboard finger schmutz maybe getting on the screen. You could also solve this problem by not using the MBA and leaving it in its original packaging and remoting into it.
Actually, finger oils transferred from keyboard to screen can cause permanent damage to an LCD if not addressed quickly enough, especially glossy ones. That's not a problem for the glass-covered MBP, but (in theory) could crop up in the Air.
This was a significant issue back in Titanium Powerbook days, but Apple's done a good job increasing the gap from keyboard to screen since then. Even back in TiBook days, I just used a damp cloth to clean the keyboard outlines from my screen periodically.
tl;dr: I used to be that guy with the microfiber keyboard cover. Don't be that guy. Just wipe your screen with a soft, slightly damp cloth when you see marks on it.