Well I think it has to do with the animals becomming fatter on grains and thus having more marbling. I've also read that there is a flavour difference, but I can't say I've tried grass fed beef or pastured chicken unfortunately.
Being a butcher doesn't make him an expert in nutrition when being a nutritional expert today doesn't make you an expert in nutrition. Afaik grain finished animals have the same fat constituents as 100% grain fed animals, so the benefits of eating it aren't there. On the other hand, if you're an animal lover it might be worth it to think that theyve been running around and what not.