You get higher amplitude.
Tests dont need to be "real world like" they need to be reviling. and we need to have the knowlage to kow how to translate re results to real world.
SINAD a 1Khz is also not real world at al but it tells a lot.
You might not get a lot of >10 kHz periodic high amplitude signals but you sure get a lot of fast rise time signal wit high amplitude.
What signal will you get through an amp, that has a faster rise than a 20 kHz wave?
The 32 tone test is done at 5 W, so at least it's not like a 1 W test (admittedly, it's not a high power test but hey, maybe we should get 32 tone tests done at 20 W or 25 W too, right?) But keep in mind, the half-way point of low frequency energy vs high frequency energy happens around 350 Hz on average, depending on who you ask. Drum & bass, some hip hop, etc. will have even more of a skew towards LF.
BiAmp (Bi-Amplification - Not Quite Magic, But Close) - Part 1
ESP - The Audio Pages. How to biamp or triamp your hi-hi speaker system for greatly improved sound, and higher sound levels with less distortion.
sound-au.com
Also, the amount of energy over 3 kHz in several kinds of music is quite low:
Music and the Human Ear
A description of human hearing, and the implications for sound system design
www.silcom.com