The method employed by at least one vendor here for their unzippable V3 bodysuit was to group a column of polys from the neck to the navel, delete that group and through the careful use of deformers stretch apart the rest of the mesh. A nursing bra with cup flaps that lower could be made from a bikini top in this fashion. Remember also that props spawned from groups retain the original object's UV mapping, so a ripped off sleeve prop made from a shirt, while losing all its clothing characteristics and custom morphs will function as a separately deformable prop. Chief reason I can think of to do this is to save memory and rendering time; one of Shamms Mortier's bigger boners in his P4 Poser book was to suggest that the ideal method for combining two figures was to hide the body of one, the head of the other, and parent to keep them together.