>What happens is that dust and sand gets between the plastic >cover and the LCD. Hard particles like sand gets embedded into >the cover because plastic is soft. Each time you walk with the >camera on the strap, the camera cover rub against you. Inside >of the cover, the embedded sand is pressing against the LCD >screen. You end up with pits on the LCD screen from sand being >rubbed into the LCD screen hundreds of times. Not good. That >is way worst than no cover at all.
This is exactly what happened with me with my D90! I didn't have a screen protector on my D40 and the LCD screen was spotless. I saw stuff on the D90 screen and didn't think anything of it for months thinking that I had stuff on the outside of the plastic. When I went to clean it much to my horror I discovered that it was between the plastic and the LCD which ended up pitted and scratched!