I'm not sure how much value author ratings would have.
I feel like story ratings are ratings of the stories in regards to what sort of story they are. So if I rate a mystery story, it's on the basis of being a mystery story; and if I see a high rating for a say, mind control stroke story I assume it's a story that does the mind control well and the stroke well.
It's a way of, especially for more niche interests, indicating how well the story caters to those niche interests.
Whereas with authors, especially those who have themes across the board, there's not really any information, I feel. "This person writes good stories"? Cool, but I care about particular tags, and authors don't have tags.
Of course, it could also be that I know I'd be at the bottom of the author rankings, so take with a grain of salt... :D