Andreas, you are drawing conclusions based only on assumptions. Jones' tests proved that quality isn't defined by a set minimum shadow gradient, but that the minimum shadow gradient is linked to the average gradient. It's a question of perception. While the shadows might be flatter, their appearance is related to the average gradient. Of course, personal taste will play a factor.
Originally Posted by AndreasT
Don't forget that with extended development while the shadow gradient is increased, so is the overall gradient and with the Delta-X Criterion, the relationship remains the same. Also, the fractional gradient speed point defines the minimum useful gradient, not where to place the exposure.
The Delta-X Criterion will produce identical film speeds for a delta D = 0.80 as will the ISO method even though the speed point is approximately a stop below the ISO's 0.10 fixed density.