When you 'weld' items some of the verts become shared, and those shared verts are allocated to one of the groups (the parent I think) If it didn't do this, then for things like full-body-morphs, you would get the verts of seams between groups being morphed twice, which makes an unseemly mess :) Haven't encountered the rendering faults you mention for welding body parts to more than one other part. This was standard practise in older Poser models. If you look at some of the cr2's for version 2 models for example, you will see that the collars are welded to the chest, neck, and each other. Since the designers did this them selves, you would think they would have made it work :) Maybe they had problems themselves though, which is why the newer models don't do such things.