This is an interesting question. One advantage might be by simplifying a rather complex period in history and making it appeal to a more general audience by presenting it in fairy tale form.
A disadvantage that I can see would be that you run the danger of simplifying the story. In essence what may be an advantage may also be a disadvantage.
Another advantage would be that by allegorizing the characters, you make them more memorable in the reader's mind.
A disadvantage to this approach would be that you it may not be apparent to the reader who the characters are to represent if they are not history scholars.
A final advantage might be that as a fictionalized tale, the 'warnings' of a totalitarian regime might be more heeded as the reader has become emotionally involved with the characters and has felt empathy for them.
A final disadvantage might be that many readers may miss the connection between Stalin and the events on Animal Farm. If a teacher were not there to tell them that this is based loosely on Stalin's reign, then many students will not know or understand Stalinism. You could not rely on Animal Farm alone to teach history.