a great book on the importance of the directivity of a speaker system can be found here

http://www.amazon.com/Sound-Reproduc...8315432&sr=1-1

it is of great importance because the room sound

if you don't have a smooth and constant directivity (in the horizontal plane of most importance) the reflected sound will affect your perception in not a good way

if you have a 12" driver till 1.5khz this driver will have a very hi directivity and when crossed to the mid range the directivity will change to a wider dispersion

in the axis if you measure it can be flat but the spike produced by the hi xo point in 12" driver will be reflected from the room

the book explains it in more educated way (and with correct English)

this affects monitor and consumer designs
jbl has a paper which you can find in the Lansing library