Some speaker manufacturers state that their tweeter’s response extends to 30 kHz, or more. Amplifier specifications often state a flat response to 100 kHz. Is such a response useful?
Well, yes, it is. The reason for this is that as the response rolls off, phase shift occurs, so the farther away from the limits of our hearing, the farther away the phase shift will occur. Phase shift can make instruments sound less “real” because their harmonics are not exactly lined up with the fundamentals.
In our youth, the upper limit of hearing is about 20 kHz. The irony is that, when we reach middle age and can afford the really high-end equipment, our ability to hear high frequencies diminishes significantly, such that the upper limit is now about 12-14 kHz. Women suffer less high frequency hearing loss than men, so we guys should certainly take our wives with us when we shop for hi-fi equipment.
But, does the fact that middle-aged adults can’t hear above 14 kHz mean we don’t need to worry about the quality of the components when we are older? Certainly not, because the quality makes a difference in the entire audible spectrum, not just the high frequencies.
So, there is your reason to continue buying the good stuff all your life.