My thoughts are that they might be used as a tool to help, for lack of
a better word "hack" your way to a better mix, as that's ultimately
the end goal.
To clarify some things :
There's no hack in mixing.There are techniques that get you there faster, but hack - no!
Also you can't just notch 2 frequencies and expect the perceived loudness to be insane. Of course it does make a difference making a V shape on your song EQ taking out some mids and boosting hi's and lo's. Also there's a classic mastering technique the crossover mastering(there could be other names for it too) which helps with loudness and control.
Keep in mind that no matter what frequency the human ear takes at loud volumes , the first frequency region that you start to "lose" or the ear itself starts to smear , or others say compress , is 4K which of course comes back after a while , but it's the first line of defense the ear has, to drop 4K. Also middle aged people that start having some hearing loss , also lose 4K first.
Sibilance and presence lies before and after 4K which are very usefull when trying to make something loud.
The parallel I can see just by visually studying the FMC, is you are
most sensitive in the 1-4k region, and that's already something I do
when mixing, try to take out the harsher sounds so you can push turn
it up louder without fatiguing.
What you state here , is that actually you want to FIGHT the purpose of loudness, you want to be able to turn up more , without it being so loud.
Loudness's purpose on the other hand , is to be as loud as possible.
So if we have a guy with a nice amp at his house , he wants to listen to some loud music , he plays your song and turns up all the way up to 7.
7 for him at that point is loud music he likes it!
Now he goes and plays another song with all the tricks that make it be perceived as louder , and turns the volume down to 5. The music is still loud for him , he doesn't even care why he turned it down.
Loudness is not a tool to let you push more when listening, it's the exact oposite.
It's an actual dogfight, a way to stick out when the radio plays at the same volume , or to drag the listeners ear when playing a list from spotify , ears like LOUD!
"hack" your way to a better mix
A louder mix, is NOT a better mix.
Loudness is only a fraction of what does something sound right , but sometimes engineers start their whole mix , in order to end up boosting the loudness , which is mixing for a purpose. You can't just go with a loudness curve and fight with those guys, you have to start building everything from the start , so the loudness curve at the end is more "efficient".
Mixing is a series of serious decisions, deep applied technical knowledge and a world of obstacles to fight, from bad rooms and speakers , to apple's mono output cause hey , guys now listen to music from the iphone speaker !?!?!?!
If you want to be loud , start accepting the idea that you'll lose a lot , musicality , harmonical content , dynamic range and others that make a song have soul.