I once knew a family who shared that view. Though educated people, the parents decided that teaching table manners was too much trouble, so they built a cement-rendered brick room where their children ate like pigs from a common dish with their hands. Occasionally the room was hosed out. In other words, like cuckoos, the parents left it to other adults to train their children. Very ill-mannered in fact.
Good manners are indeed the oil of social machinery.
They literally smoothout our social interactions. Good parents teach their children to say 'please' and 'thank you'. None of this is natural behaviour. It must be taught, and the only effective teaching is good example. Children soon see through the hypocrisy of parents whose nagging amounts to nothing more than, 'do as I say, not as I do'.