Consider these couple of situations.
When used for playback, often people want the HF to be rolled back a bit, a sloping downward response, and a bass hump.
But why? Why would you want to alter what the original artist did? They mixed it so that it sounded right to them?
But if you roll off the highs and boost the bass, you are altering the sound that the artist spent a lot of time working on.
You should want something that is a "white canvas" so that the picture the artist intended is accurately displayed.
Who are you to change what they did/wanted you to hear?
In a live situation, you have control over the tone of each and every microphone. So if the sound system is flat (a white canvas), you can accurately "paint" the tone/sound that you want.
If the canvas is already colored, then you have to "counteract" that color/response in order to get the sound you are looking for.
I go for a flat response, and love it when the engineer says to me "WOW, I didn't have to fight the system".
To me, the sound system should "disappear" as much as possible, and not present any "sound" of its own.
But I know some people like for a speaker system to have a particular "sound". I guess it makes it easier for them to pick out different systems. I prefer it to not have a "sound" and to be as transparent as possible.
Maybe that is just me