Is this a valid reason to breed?

Nowadays, I believe the only reason someone should breed is if they have a truly exceptional, purebred show quality dog. The rest are pets. However, in the good old days, it wasn't the same. People bred dogs that had great working ability, regardless of breed. So do you think it would be alright to breed a great working dog, even though it was a mutt and imperfect, for practical use? uming that the pups would be given to people that needed the dogs (e.g farmers, sled mushers etc.) This is out of curiosity, since I saw a show on Dirty Jobs about a man that bred mutts solely for the purpose of sledding.