At frequencies above 1500 Hz, the ears derive directional cues primarily by relative intensity, rather than by arrival time. So if you have an omnidirectional system, you want to make sure that early reflected energy is fairly well diffused so you don't get a distinct, image-shifting reflection (absorb too much, though, and the system can sound dull). Arrival time still plays a role in image localization above 1500 Hz, and is the primary source of localization cues below 1500 Hz, so the longer the delay between the direct sound and onset of reflections, the better the imaging will be.
Omnidirectional speakers do a very good job with intrumental textures and timbres, because they more closely mimic the radiation characteristics of live instruments. Obviously there is a tradeoff of image precision for lushness and enveloping ambience. Although I don't own or sell any omnidirectional designs, I readily concede that a well-executed omni (like an Ohm F or MBL Radialstrahler or Wolcott Omnisphere) or quasi-omni (Shahinian Diapason) is extremely enjoyable long-term.
