Thanks for the further feedback, everybody. There seems to be a near-universal consensus that image quality (sharpness, contrast) is by far the most important factor for Old Judge collectors, and that back damage is not that big a deal, but not irrelevant either.
A neat illustration of the importance of image quality over technical grade came last week, when the same seller from whom I got that AUTH card in my original post had 10 Old Judges up for auction ending at the same time, all graded SGC 10 but with widely differing images. The final sale prices correlated very closely with the quality of those images, ranging from $39.90 for this beater with a dark image:
To $61.30 and $72.00 for these cards with faded photos:
On up to $199.50 and $203.49 for these two cards with beautiful, sharp photos (both of which I bid on but didn't win):
Just to be clear, I don't have anything against getting Old Judges graded. It's not really my thing personally, for basically the reasons Jay outlined, but I can definitely see the usefulness of getting a card protected in a slab, getting independent confirmation that it's genuine and unaltered (albeit with the problems Joe G outlined, and which I mentioned in my original post), and of increasing its resale value. Anybody can look at the image (and the back) and judge for themselves how attractive it is and what they would pay for it regardless of the number grade, as those eBay results illustrate. It would be nice if the grading companies could come up with a better way to grade Old Judges, but in the meantime the market seems to be handling the situation pretty well.