One significant difference between a good recital hall and a home listening room is what's happening as far as reflections (reverberation) goes. In a good hall, there is a fairly long time delay between the first-arrival sound and the onset of reflections, and when those reflections do arrive, they are diffuse, decorrelated, well-energized, and spectrally correct.
When an omni or quasi-omni or polydirectional setup is working well, it is mimicing at least some of those characteristics of live music in a good hall. I would characterize a fullrange dipole speaker as a quasi-omni in this context, as the backwave is spectrally correct, and we need to get the speakers out into the room far enough that the backwave reflection path imposes a respectable time delay. My point being, if the problem is "how to better mimic what happens in a good recital hall", a speaker doesn't have to be a true omni to take us in that direction. In my experience, true omnis have a tendency to generate unnatural soundstage effects (ten foot wide guitars), even when set up professionally.
As for popularity or lack thereof, in addition to the somewhat demanding placement constraints, I think most audiophiles have the perception that "reflections = bad; more reflections = worse". This is true under some conditions, but not when done right, which really isn't all that difficult. And reflections done right improve timbre, sense of envelopment, and clarity. That's right, clarity... that surprised me too, but it's right there in Floyd Toole's book, with some real good research to back it up. Anyway if you allow me to classify dipoles as "polydirectionals" and therefor "quasi-omnis", once we include Magnepan they're pretty popular after all.
