I encased my MacBook Pro in one of these plastic cases - and my MacBook Air had a Speck case. In both cases, I assumed the plastic cases would retain the Macs in pristine condition.
Not so. Rather, the grit that gets under the cover, over time, caused abrasion.
On the other hand, if I did not have these plastics, who knows if more serious dents and scratches might not have occurred.
I believe the best approach would be to use these plastic cases, but regularly take them off to rinse in water to remove grit under the surfaces.