I would perhaps like to be careful to separate the questions of valuation and grading. They should not intersect at all, as I see it. The card is what it is, regardless of what someone wants to pay for it.
When it comes to the photographic cards (and Exhibit cards) as SGC handles them (can't speak to PSA since I don't use it) I see two issues:
--First, back damage: the "yes, but" approach to OJs, Exhibits and other blank-backed cards with back damage it seems to me is already part of the discretion of the grader and is already accurately factored into the grades. I have some gloriously sharp albumen cards with back damage in SGC 20 holders and I am fine with that. As far as I am concerned, a noticeable amount of back damage = poor with a bump for exceptional front clarity being part of the grader's flexibility to perhaps a 2 but no more. Sorry, but a vg card to me means basically intact front and back with general wear. The eye appeal of a sharp OJ with album removal scars is definitely more than a typical card in the grade with overall wear and tear, but I think it is already properly reflected in the technical grades up to a 2.
--Second, photo clarity: I think there is where the real issue lies. I don't think the graders give enough weight to clarity of image on the 19th century photographic cards, on the top end; i.e., I think they give a card's corners, edges and crease-free status too much weight. I don't see a very light image as meriting more than a vg grade, even if the card is sharp and clean. That said, I have seen quite a few cards from SGC that have been graded higher than I think technically merited because of an especially sharp front. This one, for example, which has a small back wrinkle but is a 60:
[pardon my sucky scan; the card is actually a lot whiter] I think it got a grade better than technical for the great image clarity. And I agree with that approach. Where I have a bone to pick is the opposite--the weak image with sharp corners.
I guess some of it also comes down to tastes great or less filling; everyone's got a view.
Pricewise, I think the knowledgeable buyers already separate the light image cards from the sharp ones, regardless of the number on the slab. That's the way it should be--buy the card and not the holder.