The ratings are what people make them. Like you said 9 is the best 1 is the worst everything else in-between lies somewhere in the good-to-bad spectrum. Let's not set concrete definitions on something that people should be able to manipulate to represent their opinion towards a piece. 

If you think it is better than bog standard average read then it will be above 5.
If you think it wasn't even of the level of an average read it will be below 5.

Basically I don’t agree with setting values to numbers which people may not agree to, at least this way with the numbers being just numbers people can set their own meanings. Though it’s not hard to see a rating better than 5 means your work is in the top half.