I agree completely. But
- there are cases where an objective anchor in the image is needed;
- having an area of known gray value can help in giving a basis for a defined exposure.
How to use that extra information is up to the photographer/editor.
And of course the reading on the gray card will vary depending on where in the scene it is. That’s why it’s used, to get a scene-defined value in the image. Again, how to use that is up to the photographer.
There are several factors in play here:
- How do I expose the raw file to get optimal information: there you have to avoid clipping the important highlights, and where a middle gray zone ends up is not all that important.
- How do I then edit that raw file to get an optimal rendition. There, a defined value in your scene gives you a known starting point.
Your exemple of a person in the shadow, in a scene exposed for the highlight is a bit of a red herring, as you are mixing two concepts: white balance and exposure. You can very well expose for the highlights with a whitebalance set to shadow. The more so as changing the camera white balance setting doesn’t change the raw file (it will change the embedded jpeg, but who cares?).
And if you expose for the highlights in such a scene, one assumes those highlights are important, so you need to keep detail there.
And Mr Adams didn’t have quite the same equipment as we have nowadays (which does not invalidate his words, but changes how to act on them).