Define "rare". Phoria's prevalence in the population may be as high as 50%. As @Failure2Stop noted while he was an instructor:
I have never recommended the use of the ACOG as an OEG, mostly because of this distinct shift we see in about 50% of shooters to varying degrees and distances of effect. I use the OEG technique only as a method to get the shooters into shooting with both eyes open.
Obviously, many people with phoria can still use occluded optic shooting to good effect at closer ranges, as they may have a mild case, and with a pistol you get much more less occlusion than with a long gun, but based on everything I know, I certainly wouldn't consider phoria to be rare, and would encourage folks to actually try out occluded optic shooting and see how it works for them, and to not be shocked if it isn't a very viable tactic for them.