The Prosecution: Changing one's mind about the quality of a given movie, or for that matter any given work of art, is a disgusting practice that ought to be accompanied by ruthless social disapproval. Re-rating allows and even encourages one to incorporate other's opinions into one's own ratings, heavily biasing them. Naïvely, many assume that this influence will always move ratings upwards and assure themselves that they won't merely follow the opinions of the most popular critics. But the reality of the conformity cycle is much more insidious: you are just as likely to learn that too many others like a movie and thus dislike it. There is no defense against these influences once you have been exposed to them, thus rating must happen early and remain steady despite the greatest of protestations. Ladies and gentlemen of the jury, I believe strongly, and upon contemplation I believe you will come to agree, that re-rating really is the bane of a high-functioning rating system.
The Defense: The vitriol of the prosecution's ad hominem attacks against everyday folks who happen to re-rate now and then, justified only by some childish appeal for purity, is dangerously short-sighted. If you don't understand a movie the first time you see it, that's not necessarily the movie's fault, it could be your own fault too. Thus it's totally understandable that, if you come to understand some angle of the movie better after conscious or unconscious contemplation, your rating might change. Moreover, the quality of a movie cannot be fully judged right after watching, because the quality of a movie is based not only on your experience during the movie, but also the value over replacement of any subsequent thoughts about that movie after watching. Thus a rating must be dynamic; it will change with the ebbs and flows of one's thought processes, the structure and patterns of one's interior life, and yes, maybe even one's interactions with other people. Re-rating is only natural given all of our other human tendencies, and its availability takes much unnecessary pressure and anxiety off of the initial rating. If we want to evolve as people, and more specifically as a society of movie watchers, then we must be willing to accept the consequences of such dynamicity. The defense rests.
The Verdict: Death by reruns of imdb's bottom 100.