The plastic snap on cases for the laptops are actually really dangerous. Every laptop I've worked on has had these plastic cases/covers cracked & broken where the laptop is literally slipping out of them. I've almost dropped a few of them because I was holding the plastic case/cover & the laptop was sliding out of it.
And dirt & grime do build up between the case/cover & the aluminum sometimes causing permanent cosmetic damage. It is also very disgusting when removing those cases/covers.
Plus those covers do not prevent damage to the laptop. I have seen corners bent, dented, and even have abrasions from the damage. At best they may prevent scratches, but that is all, however, the abrasion from the grime that gets trapped kind of negates the lack of scratches.