I don't think there is a desire for a one size fits all approach. There is a desire to quantify what makes a headphone sound good. The Harman target isn't "all headphones must sound exactly like this target" but more "this is a tonal balance in a headphone based off good speakers that many listeners prefer the most".
Which comes back to your earlier point of sticking with the designed sound of a headphone. None of these designers will have measured my ears or know my preferred tonal balance so why would I stick with the designed sound?
Well, you don't have to of course. Use whatever EQ you like. It seemed to me that EQ was just an artificial effect layered on top of the headphone's real sound. But then again, I tend to adjust color settings on a TV and turn off picture enhancements to try to reach a more natural picture, so perhaps I should do the same with headphones.
I get the sense some of them are designed physically around the intended sound of the product, whatever it is, like the pads chosen and other aspects of the design. It might all go together. The pads and foam and shape of the cups affect the sound, so that's taken into account by whoever made it that it's supposed to sound that way, and then an EQ change is not taking into account these things. But I mean you know it's up to the user to change things however he wants. It's his purchase, something he owns.
Maybe personally for me I get interested in what people hear, and what they write in reviews, so I want to see if what I hear matches up with what they say they hear. And if I'm dealing with a lot of people saying they've EQ'd their headphones to sound different, and then give a review after that, I feel frustrated because it's more like I'm reading a review of their EQ settings than the product that comes out of the box. Does that make sense? It adds to the confusion. And some of them have been around for so many years now, like HD 600, HD 25, DT 880, MDR-V6, Portapro, etc., and they kind of like become a cultural fixture in the hobby, but that changes if everyone just has their own EQ profiles. Then there's little meaning when I read that someone has this or that headphone because I realize they may have EQed it to sound like something entirely different.
But that's not important. I should probably backspace this. It's way off topic.