Hmm. I don't think that it's the edge that makes the difference. Maybe what they're saying, though, is that the halogens fade towards the edges and the xenons don't, but the edges are still the same distance from the center, so that an oncoming car 45 degrees to your side is still very bright with xenons, but halogens aren't so bright at that same angle. Is that it? The first time I read your post, I thought you were saying that the edge cutoff for xenons was closer in and the flashing between inside and outside the cone was waht was causing the problem, which is something I haven't seen.

It could be the color temperature of the xenon bulbs reduces night vision sensitivity, too. Actually, I think that makes sense, since red, orange, and amber lights are used in darkroom areas where you're trying to retain night vision and xenon bulbs are on the bluer end of white from halogens.