origanally, I bought my keyboard cover to keep my keys from getting "shiny" (it happened with my first generation AIR, and looked terrible.)
Now I realize that the cover also helps guard against spills. I usually drink coffee while I surf, and the keyboard cover keeps crumbs, spills, etc. out.
I would not have a laptop without a cover now.
Also a trackpad cover, because nothing is as ugly as a worn, soiled trackpad. And with out protection, the trackpad WILL get that way.