Why? That's not at all clear, even from your example. "If the mannequin is wrong the details will be wrong" does not imply "everyone involved must know how to do the mannequin". Just that someone must.

Let's go from figure drawing to true multidisciplinary art -- 3D graphics. At least in the gaming industry, the rigger (creator of the skeleton and joints, relative expert in anatomy) is not necessary the modeler, is not necessarily the animator, is not necessary the texture artist, is not necessarily the 3d shader expert. None of these need to be particularly good at any of the other skills.