So I come from the opposite opinion on this. I’m a neuroscientist that studies how our brains perceive all types of stimuli including audio signals and my work involves understanding the computational processes your brain does to perceive those types of signals. From a purely scientific/mathematical perspective, I would argue EVERYONE absolutely should use EQ (given the EQ software is competent and the headphones in question can handle EQ without distortion) for all their headphones.
The reason this is necessary with headphones is because we each have our own individual head-related transfer function (HRTF). This is unique to each person given your own physical biology/anatomy as well as the way your brain has learned to process sound. With speakers, the reason we can measure “flat” or “neutral” is because both ears are receiving sound information in conjunction with how it resonates and reflects around the rest of your head and torso. With headphones, you are instead forcing sound individually to each ear independently, and therefore, “hiding” important sound cues from each ear. So because of that there is no true “neutral” or “flat” for a given headphone since it is entirely dependent on your ear shape and your HRTF. Check out Tyll’s lecture called “Finding Flat” as he explains why when you look at raw FR graphs of headphones you specifically DON’T want them to measure flat.
Next, I urge you to watch David Griesinger’s lecture on binaural hearing and equalization. The link is queued up to the most important part of his talk, though I recommend watching the entire thing. He has calibrated 10 different peoples’ HRTF response so he knows how each person will perceive sound from a given headphone, and he then played each of them pink noise using HD600. He then demonstrates with samples exactly how each person hears the same pink noise being played to them and you can hear how dramatically different each person’s response is. It is a very striking and concrete example of understanding why each person should be using EQ to precisely try and hear what the original sound engineer had intended.