I couldn't agree more -- there are lots of things that you can't check from a single angle of lighting or viewing, luster being the most obvious. On the other hand, the Internet being what it is, it's useful to practice "grading from pictures", and it is possible to get better at it. No, you'll never beat someone who's got the coin in hand. But if you're checking out (say) a coin on eBay, a picture is all you've got.