In my twenty plus years of shooting and developing film I have learned a few things (still have much to learn). One thing that I have learned is that if one follows the suggestions of Ansel and of Fred of establishing .10 as the film speed point that it virtually guarantees prints that exhibit empty, flat, and unseparated shadow tones. That is I assume why some have taken to placing shadows at III and IV levels. Because a .10 density and incorrect placement will guarantee that shadows fall on the toe of the film's characteristic curve. For shadows to have separation they must be exposed up off the toe of the curve. The other thing that I have learned is that developing film to a 1.25 to 1.35 above FB+fog (per Ansel) is that the prints will exhibit very poor local contrast. The overall (general) contrast may be acceptable but the local contrast is seriously deficient. In my experience it is the local contrast within the print that brings life to the print. It is not the overall contrast at all.
Think about this for a moment, if you will. If we test and establish that a film such as TriX, for instance, is truly a EI 160 film (based on a .10 density in HC110 dil b). Then we place the low values in an exposure on a Zone III or IV. Aren't we in effect saying that irrespective of the manufacturers rating or Ansel's and Fred's edicts that this film needs to be exposed at EI 40 (Zone III placement) or EI 20 (Zone IV placement). Then when we have in our development testing placed our high value densities at 1.25 above FB+fog we have in effect shortened the entire scale of the film. This is a sure guarantee for flat and lifeless prints in my darkroom.
What I find today is that the negatives that I exposed based along the lines of Ansel's guidelines are not printing the way that I want them on grade two paper. (Not Oriental Seagull---not anybody else's). I end up printing those negatives at anywhere between a grade three to grade five filtration. What this tells me is that the density range of the negative (using Ansels guidelines) is too compressed.
The fact is that the EI of a film is a moving target. It is not stationary in the case of plus or minus developing. Nor is it a stationary number if one switches between different developers. It may be and then again it may not be. Yes we need to have some type of guideline in exposure. In my estimation the principles that Phil Davis advocates more nearly approximate the characteristics of our materials in the real world. Ansel and Fred did no testing of the exposure scale of the papers that they were using. Phil Davis considers the paper as the primary starting point for the entire photographic performance. It makes sense to me that if I am about making prints and not about making negatives that I would want to determine the paper characteristics and from that point then bring my negatives into accordance with the paper. This is because the paper is fixed in it's characteristics (in the case of a given paper at a given grade for that paper).
I think that if we didn't use VC materials nearly as much as we did today. There would be more people unhappy with their results using Ansel's and Fred's guidelines.
As others have said this is about producing prints that exhibit the tonal representation that we wish for our efforts. For myself this is not about following the guidelines of another photographer especially when their guidelines are in serious error.