Plan 9 is rated too high at this point. People seem to have been giving it 'good' ratings because they are finding it entertaining to watch because it's so 'bad'. The rating system is being abused, somewhat, it seems. Troll 2 used to also be lower rated, I believe, now it's in the bottom 20 instead of the bottom 10, for the same reason.
The ratings scale now seems to be used to rate 'entertainment level', which is another matter. There is no way that Daddy Day Camp can be worse than Plan 9 or Troll 2, yet Daddy Day Camp ranked 3rd worst on IMDB as of the time I posted this reply. But perhaps Troll 2 is more entertaining, based on the ratings, even if a much worse film.
-Chris