I don't follow your reasoning. If I'm having trouble hearing details, such as words in a song or the notes of a chord, it helps me to turn up the volume. The flatter curves at higher volumes suggest that different frequencies have more consistent apparent loudness when the volume is higher. Consistent loudness would seem to help the listener hear detail. What I think is going on when the volume gets high is that both our hearing apparatus and the amplifying apparatus can become overwhelmed, so signal gets lost in noise. If you graphed the level of perceived detail on the y-axis, against the sound pressure on the x-axis, I think you would see a hill-shaped curve, with rising detail at low-to-moderate volumes and then falling detail as the volume became too great.