I like using 90° flares in cornerhorns because the angle of the horn flare matches that of the walls. That's really an ideal solution because the low frequencies are directed by the walls and those higher up by their horns. Everything is matched and the reverberent field is charged uniformly.

Of course, not everyone can use this placement, and when the speaker is placed out away from the walls, then there are reflections to deal with. If the coverage angle includes an incident wall, then early reflections might make narrow coverage a tempting solution. Some pull speakers away from walls, to reduce early reflections. But either way, I still don't like a narrow coverage pattern. Reflections are controlled, provided the horn isn't pointed at a wall or very nearly. But the listening area just becomes too small. The reverberent field is usually non-uniform too, since the highly directional nature of a narrow HF horn is very different than the non-directional LF energies produced by the speaker.

By the way, why do you not like the term "dispersion"? I like staying away from subjective terms as much as you do, because they're just too ambiguous. But I don't see anything unclear about "dispersion", unless you want to be more specific as to what the cause is for the pattern. I did not, since I was looking for an open-ended discussion on high-efficiency speakers in general. Some might like slot devices, some might like constant-dispersion horns. Others will like round horns and some won't like horns at all.

Wayne