I agree that if everyone always treated a stopped propeller as though it could start the engine if moved at all, live mag checks would serve little purpose, other than to confirm "airworthiness" of the mag switch itself. I also agree that the need to hand prop planes is happily becoming less, as non starter equipped planes become more rare. But there can always be reasonably legitimate reasons to pull a prop through by hand, like circulating engine oil after preheating, before starting, so with careful live mag check technique, I'd rather do the check, and have more (not total, more) assurance that the system is airworthy. I will still treat a prop as though it's live all the time.
'Story from the past:
In a past life, I was the O-200 key clutch repair guy, so all the clutches that cam into the engine shop for repair came to me. One of our customers, with a C 150, called back to say that the clutch we'd (I'd) just sent back repaired would not engage, and he was a bit fussed about this, wanting to go flying. I was told to fly down, and make it right. So, I packed up my tools, prepared to rebuild the clutch on site. It was possible I'd made an error, and it would not engage, so I was ready to make it right for the customer. I arrived to his plane after an hour flight, it was the one with the top cowl beside it on the grass. Sure enough, I turned the key, starter ran, prop didn't - problem. I asked the owner what had been done for installation. He replied that he had simply put the clutch back in, and had not fussed with anything else. So, I took the clutch out, and checked it, it seemed to be working correctly. I reinstalled it, and did some more checks, and everything seemed to be working correctly, I could feel it engage and disengage the starter motor with slight, and careful rotation of the prop. But still, the starter motor would not turn the prop. In diagnosing, it got to the point where I had the owner engage the starter, while I wiggled the prop with a bit of light rope, intended fly off the tip if the prop started to turn, it didn't. I was puzzled, and this was getting less safe to experiment with, as everything seemed to be working as it should. Then in desperation, I re-asked the question I'd already asked, just a little differently: "Are you sure that you haven't taken anything else apart here?". Then the reply (he knew I was really frustrated now); "Well... I did take the back plate off the starter motor to look inside...". The bugger had put it back on 90 degrees out of phase, and the starter motor was turning backward, of course my correctly repaired one way starter clutch would not engage! A quick fix, and the whole thing worked just fine. I could have saved an hour of frustration if I'd known to check the starter motor for misassembly. I must have looked like a fool wiggling a prop tip when the owner held the starter running!
It's worth understanding how your starter should work, and getting it fixed if it doesn't. And still, I support careful live mag checks....