First, they are not "plastic" wear rings, they are made of a very hard material, almost like a super hard nylon (would be my best description)
If your impeller is set up correctly, you need not worry about an additional wear surface. It is my understanding that these seal much better than a metal one because they are able to flex just a bit and seal around your impeller better than metal. This is my understanding anyway, all I know is that they give you a tremendous boost on your hole-shot!
I've had one in both of my boats over the last 7 years, and have seen no wear at all.
Looking at this in reverse, I'd much rather have the impeller up against nylon than brass if things did get out of sinc. Personally would rather buy a wearring than an impeller.
Ken F