I doubt it is much of an issue with most normal viewing conditions. I have never seen the phenomena described in what I would call normal viewing conditions. I wondered about that quite a bit. I read edz's last post on Cloudy Nights where he commented about reflection from the retaining ring on the objective and from light coming in from above the binocular. When I was reading that post, I had a thought. I use Bushwacker flip up objective covers on both my Promaster and ZEN ED binoculars. This creates a bit of an extension, sort of like a sunshade, which may block the light edz talked about coming from above the binocular. So maybe that is why I do not see this, and it might be a potential easy fix for those who are bothered by this. Those standard objectives need to be replaced anyway as the do not stay on worth a hoot.
However there is still something I have not grasped quite yet. If I take the Bushwhackers off, then I can see the effect that has been described,but ONLY if I place my head at angles which I normally would not use. I actually need a pretty grotesque off angle posture that would kink Paul Bunyan's neck. Almost like looking through the bottom half of the binocular EP with the top half of my pupil. So I don't yet readily see why this is not at least partially eye position related.