I think you can accomplish such a functionality already using tags. For example, why not rate articles as "1 star", "2 star" or "3 star" or as "top tier", "mid tier", "low tier", or whatever exact system you choose? Then, you can simply filter the articles by your importance tags.

I still think a column with a star-rating system would be a nice addition (and easy, one-step solution to rating an article).

This would be very helpful and better than my current rating system which is based on tags with varying numbers of stars

Monicab's suggestion of using the Extra field works well. The only problem with it is that if I want to do an annotated bibliography, the Extra field is where I write those. So, if you wanted to be able to rate them and include an annotation you might run into an issue. I still think a star rating system would be very useful.

Colour code tags work quite well--e.g., primary source red "01", secondary source green "02", and tertiary source blue "03". That way, the tagged citations are colour-coded when listed and unrated citations can be easily rated by selecting them and then typing the colour associated shortcut key 1,2, or 3, in this case.