Say you have a rig that you made a template.
You are animating said rig and say you add a new drawing. With pose copier, there’s no problem with that drawing. However, if you add a deformeration to that drawing (and ad deformation already exists for that structure, like say a hand), and then use pose copier to do a body turn, the turn will break that deformation and also reset any deformer on the drawing leading up to when pose copier was used.