The reason I have been given is that it will make it easier for a poser figure to be used in other software.  To a small degree that makes sense in that the border between body parts would only be one vertex instead of two identical (or very close) vertexes.  That way you could take a morphed figure it into say Blender and make more detailed changes and be able to use that model as a morph target that has the correct vertex order.  Problem with that is the Ken1171's script Uni-mesh exporter for P12 does that so makes it a moot point.  Another is that rendering would be better as there would be no seam breaking apart between body parts.  Although in that case it would be a matter of simply welding the parts together to send to the render engine since it makes no difference if you lose the exact vertex order as it is meant to be tossed after the render.  The third case would be Posers Morph brush would not hiccup on pairs of vertexes between two body parts.  Again there is a work around although a bit more complex.

Poser 5, 6, 7, 8, Poser Pro 9 (2012), 10 (2014), 11, 12, 13