The muzzle wear pertains to the extent of bore wear done at the muzzle from cleaning. How much the wear affects accuracy is hard to quantify, though if a muzzle is worn out, it isn't going to shoot well.
I don't know much about the field grade rifles. I bought a Service grade rifle a couple of years ago and its an outstanding rifle, worth every penny.
As for the rack grades, I think most people buy these for 2 reasons: To fill a niche in their collection, or they have plans of rebarreling them. Some of the rack grades are going to be good shooters, and probably some of them are shot out- kind of a pig in a poke chance when buying.