I think beginners often need some black and white rules to get started. I found that to be true teaching math and science to college students and it seems to be true for newer soap makers too. Parents put training wheels on a beginner's bike, and take them off as the rider builds their skills.
Some of the rules that keep floating through the Forum seem to be rules specifically to simplify things for newcomers. Some examples -- The INS of the "ideal" recipe is 160. Fats and lye temps should be within 10 degrees F / 5 C of each other. Soap with "full water" because a "water discount" is only for experts. A 5% superfat is the lowest you should go for safety. Etc, etc.
When you rely on a single number -- INS or iodine -- to define a complex system, you're basically dumbing down the decision making process. That might be okay for a quick-and-dirty check or it might be good to help a beginner get started. But it's not good if you are trying to dig into the nuances of why one soap performs more better
INS is the equivalent of playing a simple C major chord every time you see any chord that's based on the key of C.
I could create two recipes with the same INS number but using different fats. I guarantee even an average bather would be able to tell the two apart. One example would be a 100% lard recipe and the other would be a blend of 22% coconut and 88% olive.
The lard soap will last a long time, but it will produce smaller amounts of dense, long-lived lather. The coconut-olive soap will have a much shorter life in the bath but will produce a larger amount of fluffy lather.
I know this from experience, yes, but I can also tell this just by looking at the fatty acid profile. I can't tell this by looking at INS.