Martin, you are certainly correct about the degree of flexibility in the developers and the ability to tolerate all mixes of films with some degree of average good quality.
But then, the other side of the argument is that all films made for C41 are tested for the real C41 developer formula, and that formula tends to mitigate unwarranted deviations that might be present in other developers.
For example, mixing a 5% iodide high speed film with a 1% iodide low speed film in an iodide free developer may result in different side by side 'contamination' effects between films that the presence of iodide in the developer is intended to damp out. You see, each halide can be considered to be part of a feed back loop or dampener for excesses in reaction.
So, the correct C41 developer contains KI at 2.0 mg/liter and that is a VERY precise amount. It acts as a 'dampener'. The pH is 10.48.
It contains polyvinly pyrrolidone and diethylene triamine as well for other effects such as neutralizing problems that arise from using municipal water supplies. I mentioned this above.
I admit that some of these deviations in response might be acceptable, but even in a hobby, the cost of film and chemicals is not insignificant, and the time to take the pictures is not worthless either, let alone those pictures that can never be reshot. So, although I have the formulas and chemistry, and I think mixing the chemistry is elegant, I use prepackaged Kodak chemistry. Fuji-Hunt is just as good, but harder to get here in Rochester. (I wonder why??)
Anyhow, having found one error in a formula, or two, or three, do you wonder what else lurks there? I have found, in practice, that the real formula 'creeps' even from Fuji-Hunt and Kodak, due to product 'creep' and the introduction of new technology. For example, at one time color paper used Calgon and hydroxylamine in the developer, but Calgon changed their product formula so Kodak switched to Quadrofos, and then they found that Diethyl hydroxyl amine oxalate was easier to handle so they switched to that. This resulted in a tiny correction in the paper formula and the developer formula that was gradually introduced so that no upset was seen by the customers.
In particular, IIRC, it was introduced regionally, and major customers were informed of the change. Tech Reps stayed on top of the changeover to insure quality and uniformity. This has happened several times over the years.