For me I have trouble giving a star rating at all, so I go with the more positive approach of why you should see a movie. Just because everyone might give 'Saw' and 'Nanny McPhee and the Big Bang' 4 out of 4 they are very different movies and each has a very different audience, so it is more useful to say see 'Saw' because it is one of the greatest horror films, or see 'Nanny McPhee and the Bag Bang' for great entertainment for the kids.
Stars are handy for a quick gauge, if you tend to agree with the reviewer's views generally, I just can not give them any more.
PS I have never seen Saw, all ratings are just for demonstration