Is a thin silicon case for just the TOP portion good for MacBook Air protection?

Any snap on cases will end up trapping dirt & debris beneath it. And that dirt will be ground into the aluminum causing staining & scratches.

Plus, every time I see these covers on a laptop I'm repairing, the plastic case is nearly always cracked. There have been multiple times where I almost dropped the laptop with one of these cases because the laptop slid out from the broken plastic snap on case.

Plus, getting these snap on cases off the display assembly is very difficult. You may actually end up cracking the screen as some of them can be quite tight. These days I will ask the user to remove these cases if it is a personal device because I don't want to be responsible for causing damage to the laptop. At least if the user damages the laptop while removing the case, then I am free & clear & can charge them for the extra parts.

I can tell you the grime beneath these cases can be very disgusting especially if moisture has gotten in there as well.