I'm of the opinion that scratches and dings mean nothing as long as the coils inside are not exposed to moisture. The poly covering is simply the case to protect and hold the coils in proper alignment and has nothing to do with the electrical interaction of the coils.
I hunt the desert where there are plenty of sharp rocks and zero grass. My 6x10 Eclipse DD coil sure takes a beating and the bottom looks like someone carved a road map into it. However, it remains as sensitive today as when I bought it two years ago.