Personally I think it's a rather stringent rule. Maybe for the older stuff, yes. But for modern posters, a stock pic is not a dealbreaker for me.
Dale uses stock pics for quite a few listings, but when he says they're "mint", I believe him.
I agree that in an ideal situation sellers have the time, energy and resources to take pictures of every listing they have. But really, even with a good set-up and quick hands, it still takes 5-10 min to unroll, lay, snap, remove carefully, roll, then put it away. Rinse and repeat. Even for 100 listings you can tell how it adds up very quickly.
Another thing is, sometimes there are some slight dings or nick that simply cannot be captured on a camera, unless you're using a professional 24mp camera, a sharp lens, working with a 15MB RAW file. For a $5 poster, I honestly wouldn't go through that trouble.