I find the exact opposite of you to be true in my Home Theaters. The higher directivity sources make the system image more stable, even the center channel, and with less wall reflections the sound is less colored. Most who experince my theaters are impressed by how well the image quality holds off center. I see no reason why an omni system would ever image well off axis.
I only use omni speakers for the surround channels where image and coloration are of a far less important nature.