I find it quite a practical idea for field use, if treated as a simple rule of thumb, not gospel.
Not all photographic situations require a highly detailed and thorough theory to pull off the shot. Often it is quite enough to have a vague notion of how to approach something.
"I need more depth of field, stop down a bit. But careful about going too far and hitting diffraction softening".
Works a decent amount of the time. Something that is easy to remember and covers a wide range of use cases is good. Nice and simple, especially for photogs who are not big on theory and just want some handy rules of thumb to help them.
I've seen tourist photos improved when they start focusing on infinity instead of focusing on the subject and trying to stop down to capture distant features ;)
When you and I learned about exposure, the ISO setting was called the ASA setting, and it was set when we loaded the film. For most of us, the only settings we ever used were 64, 100, 200 and 400. And we didn’t think in terms of noise, we thought in terms of grain, which was a property of the film itself, and I don’t think it got much more complicated than that for most of us.
And maybe it was more complicated than that, but most of us didn’t deal with that level.
I would go even further, saying the development was guided by the film type and available exposure; and that some outstanding photographers ignored lab / printer problems altogether.
As I said, sir, we set it when we loaded the film. If we were going to push the exposure by a stop or two, we still had to set it when we loaded the film. And we set it by turning a dial labeled “ASA,” not “Exposure Index.”
So how does that translate to current digital reality? One very simple answer could be that an ancient ASA shooter who just copied what was on the box to the camera (my first SLR already did that automagically and the mini lab did the rest) would advise to put it on auto-iso, but somehow I expect that will not be your answer.
I put the film in the back or into the film holders before I left for the field. I had no idea what I'd encounter in the field. After I saw what I was up against, I decided on the development I needed, which changed what I dialed into the spotmeter. Calling EI "ASA" was basically lying to the meter, but it worked out fine if you used the meter to place tones.
The analogy for digital, where the sensitivity is set when the sensor is manufactured, would be to set the camera to base ISO. Everything else requires a push.