These are great for people who are still learning the frequencies. Sometimes when I'm in trouble I might look at the "magic frequencies" website. But ofcourse obeying this 100% can lead to some bad things like low passing guitars at 6000hz which might not always be a good thing. But if everyone remember to put that lowcut around those frequencies to those instruments I'm sure we'd have way tighter mixes in general.
The most I get out of this is some terminology, doesn't really teach what should really go where, just where the fundamentals and harmonics are, which is still very nice though.