The book you need to read is Sound Reproduction by Floyd E. Toole now in its third edition. It will answer a lot of your questions.
Flat frequency response
I am often surprised by the number of speakers with "gee-whiz bang" technology but can't even get speaker design 101 right. I can see the benefit of avoiding a lot of signal processing but preferences notwithstanding, flat frequency response seems like the logical place to start and then progress from there.
1) Why is it so hard to achieve?
2) Does it matter?
3) Is it reasonable to say when you skip the basics you are only progressing on a flawed foundation.