I think that number ratings can prove useful when there is an objective criteria that they are summed from.
For instance, simply saying "7/10" really doesn't help the artist/creator/writer unless they wish to know how much they liked it (which, in this case, would be 70% "likeness).
The solution to number ratings are those backed up with criteria. For a MOC, there could be a few categories that each serve a part of the total number rating, e.g. "visual appeal," "construction," and "variety/originality."
With separate number ratings for each category that add up to a greater total rating, I believe I can justify using them. However, I must admit I am rather lazy when it comes to things like this. Any post that would delineate the goods and bads of a piece of art/story/creation obviously shows effort and appreciation of the artwork. The number ratings would then just give some objectivity to the subjectivity of art. ^^