The "theory" is that omnis are actually easier to locate because they spread sound about the room more evenly and the location of the listener between the speakers is not as critical. In my own listening, I find that one does not have to be so tightly locked into the ideal spot to get a good stereo image--even when one is sitting one line with one speaker, the sound from the farther speaker is still quite audible and there is a credible stereo image. But, as with ANY speaker, there is still just one narrow, ideal location, for the listener.
The best sound I've heard from the likes of MBL and German Physics omnidirectional speakers were in quite large rooms. But, they still sounded quite good in smaller rooms tooo, so I don't think they are particularly picky.
As to the odd comment about omni's playing out of phase, I can only guess that someone mistook dipole speakers for omni's, That alone should say something about the merit of that comment.