I don't work in the recommendations side of the house, but the way I understand things is that nowadays your star ratings (and now up/down ratings) are primarily to learn about you, what you liked and didn't like and therefore what you might like to see in the future. They're not really about creating a Rotten Tomatoes or IMDB style database of trying to rank all movies on some sort of objective "regardless of what you like, this is a high quality movie" sort of scale.