I use the thin clear Moshi covers for both my MBA and Mini keyboards, and I like them.
Dust is an issue for at least half the year where I live, which is the main reason I use them. (Also the reason I will be building an air filter box for my Mini soon.)
Plus the cover keeps keyboards clean from your finger grime, which helps when resale time comes around.
The cover material (polyurethane?) does 'dirty up' and discolour fairly quickly from skin oil, which is hard to avoid. I use a microfibre screen cloth between the keyboard and screen when I close the lid so the keyboard cover doesn't ever touch the screen and leave marks on it. Works very well. The screen is still very clean 18 months after buying the MBA (new). You do have to remember to lay the cloth the same side up each time. ie The 'clean' side up.
The cover does sort of muffle the sound output from the MBA built-in speakers to a noticeable degree, so sometimes I have to take it off to hear stuff properly. I also have some nice headphones to help deal with that, but they are not wireless so they are not always practical.
Never had a problem with over heating on the MBA.