When you rate a movie one to five or one to ten, you start with unwatchable and you end with perfect cinematic achievement. This tends to put sweeping dramas in the top tier, low budget sci-fi and horror in the bottom tier, and passable indies in the middle. What happens to movies that are objectively bad, but incredibly entertaining? The charming, yet awful? The spectacles of failure that we watch ten times more often than our "favorite" movie?
This is why we use our own rating system. It was developed specifically for horror movies, but we've found it works well with all genres and can (and should) replace whichever rating system you currently use. We punish mediocrity over failure, so our scale goes from positive three to negative three. The positive spectrum is for good-good movies, the negative spectrum is for good-bad movies, and zero is for movies so unremarkable as to be unnecessary.