If you want to use omni's you need a lot of room.

False.

I owned MBLs in a 13x15 room and they worked great.

among hhe best sound I ever experienced were big MBL 101D in a shoe-box sized room- well treated.

shoe-box sized room- well treated.

Well, that's kind of the same thing I was meaning.  I meant that omnis need to have as few early reflections as possible, either a big room, so they are far from rear and side wall,s or the room needs to be well treated. :) My bad for not being clearer.

No doubt full omnis like mbl or German Physiks work best with distance from walls. Room needs to be able to accomodate that. The key always whether omni or not is to manage early reflections which blur  the sound. Treatments in a smaller room might help with that.

Another example is Ohm Walsh which is pseudo-omni by default in that the drivers are attentuated with absorbent material inside the driver "can" which has similar effect and allows placement closer to walls which works best for more people.

Larger rooms need larger speakers for full range in general whether omni or not.

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.

I have a pair of Ohm Walsh Sound Cylinders. They would sound great even in a closet! Sort of like a giant set of headphones!