I scoured meta and have not found any discussion on this topic.
I thought about it when I very much appreciated an edit someone made http://askubuntu.com/revisions/146161/2https://askubuntu.com/revisions/146161/2 which greatly clarified my original title http://askubuntu.com/posts/146161/revisionshttps://askubuntu.com/posts/146161/revisions
And then realized I often would have upvoted edits in the past as well, I don't see downvoting as being so useful though because you can really just roll back or re-edit if it is that bad. So, downvoting becomes a little convoluted.
But, I do think, allowing an upvote/downvote next to each edit may:
Encourage good edits
Encourage more incentive to make edits
Discourage bad/careless edits
All in all, I feel it may be useful. Not so much a feature request as wanting to understand why it doesn't exist. Any thoughts ?