Exactly. Heavy metal uses those frequencies, AND REQUIRES THE UPPER HARMONICS. Other music also requires it but you can get along without it if the array has all the other good things.
This is how Jim Griffin describes it:
"Comb lining will dull the highs and will be most noticeable in the lack of air (dull highs) especially as you move off axis. Essentially, the high frequencies are rolled off as the ensemble of the tweeter outputs don't add up but start to cancel. Now the ear is less sensitive in the upper octave (10-20 kHz) you may not observe the full impact of this effect. But if you compare to an array that is properly designed, then there is no comparison as the proper array will yield a more airy (and accurate) sound."
Jim Griffin on Parts Express Tech board on 11-21-06