You won't get many working bred dogs that are KC registered. I totally agree with your last paragraph, although almost all of our own dogs are ISDS registered, however we have had one or two unregistered dogs who were cracking workers with great health/temperament so wouldn't completely discount them either.
Ahh yes actually I am mistaken, I thought my friends dogs were but then I remembered them naming them for agility - so you are right - none of them KC registered, some of them ISDS registered - and a couple I know were from registered parents but the farmer would only do the pups if the new owners wanted it cos it ment a trip into town and he really lived in the middle of nowhere
We told the shepherd we got Tip (the pup with ED) not to bother registering him, as it would have cost him money and we definitely have no intention of breeding/trialling him.