Hmm, thanks PhilC, that may well be, but in the first instance I noticed this, I created a single vertex morph using a magnet on one of the seam split vertices and saved to the library so I could track the vertex number in the morph of that actor back to the original vertex in the obj file save by poser with the figure. There were exactly two vertices with identical coordinates in the object file matching the morphed vertex. 

IIRC, there are actually two modes of "welding": either the vertices within a specified tolerance are merged into a single vertex, thus breaking any morph targets, OR the normals of vertices at object boundaries (our weld seams) are unified, I.e. made identical. 

With the right lighting conditions, it's quite possible to spot torn seams because the render or preview shaders react differently to divergent surface normals at unwelded edges. Perhaps because the object file is not specifying normals, they're being incorrectly calculated. Or, more likely, there are divergent normals preserved in the obj file that are preventing Poser from performing the expected welds. I.e. divergent normals "means" don't weld to Poser. Double Hmm. Back to the obj file. 

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