At first it is clearly useful - hearing flaws helps fix the big issues and makes the system more enjoyable. But once the fundamentals are right, does sharpening the ear further still help, or does it just turn into a distraction?
For me, I believe the answer to your question is really one of practicality. Generically it goes like this: "what do you intend to do with your better-trained hearing system?" For me, this has been multifaceted over time:
- In my childhood, I had to be able to hear issues with amplitude response of DIY loudspeakers I built, as well as pick out electrical contact issues and amplifier issues because measurement microphones were way too expensive, and measurement analyzers were completely out of reach economically in terms of DIY audio.
- Later at University, music school ear training was necessary be able to pick out melodic lines/chords to be able to improvise and/or transcribe music by ear. (Nowadays, there are relatively low cost PC apps that can do the transcription task quite well, but improvisation still requires a trained hearing system).
- Much more recently (the last decade or so), as I discovered issues with mastering of some existing music tracks I had, then embarked on a long-term demastering task of most of my CD tracks. Looking only at cumulative spectral density and spectrogram plots isn't enough to figure out how to "undo" some of the obvious mastering EQ (clipping, of course, is the first thing you see on the time-amplitude trace)--I had to be able to hear the differences using only my ears. Being able to hear 1 dB differences in music track bands is required to be successful in doing successful demastering--just like the mastering guys themselves develop this ability over the years. Presently, I've got well in excess of 10K hours of using my ears for this task--since I discovered that most all music tracks (greater than 95% of the tracks I own) have mastering issues, so my ability to hear loudspeaker amplitude response issues as well as track EQ issues has improved greatly.
Understand also that in order for these demastered tracks to be truly useful, you've basically got to have near-perfect stereo/multichannel setup to avoid having to demaster all tracks due to amplitude/phase response issues of your setup.
- VST plugins that can undo mastering compression, i.e., multiband expander/compressor and dynamic EQ, which are applied after clipping is repaired, are now relatively cost effective, and can now be used in real time during playback of dynamically compressed tracks (i.e., virtually all released popular music tracks are compressed). So training the ear and the eye to detect which existing track bands need expansion is a learned skill.
So these different activities required different types of ear training.
As far as pursuing esoteric "golden ear audiophile" type hearing capabilities, such as phase reversal in loudspeakers (i.e., being able to hear if the leading acoustic wave from the loudspeaker array has positive or negative pressure), and other esoteric "hearing tests" found on test CDs put out by sources like Stereophile in the early 1990s, etc...I never had much use for these capabilities, so I didn't spend time trying to improve my ability to hear them. For me, that's pretty much indistinguishable from what I see in others as "overt OCD symptoms". I've not been subject to those kind of compulsions in the past.
And there is a thornier side to this. Research shows that individual differences in auditory ability are not only about training or experience - genetics play a role too.
When I was in elementary school (6th grade), I remember a test performed on those students wanting to take band (or choir) the next grade year. What the tests were looking for was tone deafness. I've found that if you have pretty strong ability to pass that test, you probably have most of the abilities you need to do ear training--at least to the levels that performing musicians need to be successful.
Of course, the more talent you have, the easier it is. I met a few students in music school having
very strong absolute pitch capability (something in retrospect I'm glad that I didn't possess to a greater prevalence, due to the problems that come when trying to play with others on instruments). I can detect issues in tuning that others might call very long-term
tonal memory--being able to tell if a group of musicians are uniformly playing flat or sharp--without first being cued--or on a
different basic pitch standard, e.g., A415, A440, etc., without having to hear both cases of orchestras playing the same piece to different standards.
I can tell you that a strong absolute pitch capability has significant downsides when playing music with others--because the typical accompanying synesthesia can be a big challenge to get around (...and not color synesthesia or "chromesthesia" in my case, but another type...). The one with absolute pitch hears the notes that everyone is playing as "wrong notes". Stop and think about that for a moment...
So where is the right finish line for non-pros who mainly listen for pleasure? Is it enough to reach a natural, balanced sound - whatever that means to you - and then stop for a while, until the next wave of technology offers a real reason to move to the latest generation of gear? Sure, for many, the cycle of refinement is part of the hobby too.
For me, this question answered itself: I trained my ears to be able to do the tasks that I wanted/needed to do.
Today I listen for pleasure, too, but you may classify me as being in a different category--based on your qualifiers that I quoted just above.
Could I have kept going in ear training? Certainly. Did I need to? Not really. I got the skills I needed--to the point that I found that I was passing Alton Everest's and JBL certified listener tests without error (the first time I took the tests).
Chris