The summation of many directional instruments may well produce omnidirectional sound. Strings will be more omnidirectional.
Omnidirectional speakers are probably not best for vocals. They are not movie speakers.
If the direct sound from the speakers reaches your ear sufficiently early, before the reflections reach your ears (I cannot recall how many milliseconds difference is needed... 5ms?) - then your mind filters out the reflections - you end up hearing the imaging - it can still be pinpoint imaging - and then the reflections arrive and provide the wonderful ambiance - and YES it works very well indeed for Movies.
But - with standard monopoles - you can place them close to the room boundaries (relatively, depending on design) - allowing for a smaller room
With true Omni's, you need at least 2m from speaker to walls in any direction - so you need a very large room
If you have a ballroom ... this is not an issue
For standard homes, it can be an issue - and if due to lack of space, you place them closer to the walls (ie: reflective surfaces) - then you start to sacrifice your imaging - and they become less suitable for Movies as well as Music - imaging and sound location are just as critical to one as the other.
BiPoles/Dipoles can be easier to position, as they are less sensitive to the side walls
Alternatives are to use diffusers and absorbers strategically.... but keeping in mind that the reflections are a positive thing - you want them, but you want them suitably delayed.