You're right of course, in the scheme of things climbing is of minimal significance compared with the other agents of damage. But I don't think "other things cause far more damage" is a good reason not to talk about a particular problem (I've heard metal detectorists hide behind that claim a zillion times) and this thread is about the particular issue of climbing after all.
In any case, it's more significant than the damage it actually does and IMO as enthusiasts we have a duty to frown on it. If we don't react against it then who will? Every year we have EH allowing (yes, allowing) the world to see drunks climbing Stonehenge. It's a shame if TMA of all places reinforces their subliminal message that it doesn't really matter. As you say, "Does this mean we should all go and climb up the nearest standing stone to look for cupmarks? Probably not".... then let's say so, not to the self-certifiers that do it but to the public who could do with getting the right message not the one from that lot or EH.
Nigel, i don't think anyone is defending climbing every stone you visit to inspect it for RA, take the Avebury stones for example, there's no need to, do your research first and generally find someone's already given such stones a thorough going over, Professor Terence Meaden for example.
I understand in the case of the DD that there were no photographs of the top of the "capstone" hence validity in climbing it.