It is true that most of us can hear 15kHz top, but there are harmonic freequencies present in music or any audiable freequencies that will go bellow 40Hz and above 20kHz. Ability to reproduce with presicion these harmonic freequencies will better define a natural sound.
Audio at 54 Khz by Tannoy
I was arguing with one of my friend today about the Tannoy super Tweeter witch can go up to 100Khz of frequency at -18 dB. But my friend was saying that this tweeter is for dumb people because the humain ear cannot go over the 20Khz frequency. I didnt agree with him but he really took out a good question. Any opinion?