Multibody sheet metal is not something I've had the chance to use a lot. The little of it I've used it seems a little awkward. It's not clear to me what it is meant for. The fact that you can't flatten multiple bodies seems to mean that they don't want you making single part files with multiple finished parts in it. So maybe they meant it to be a transitional state or to work with configurations.
Is anyone out there using multibody sheet metal? What are you using it for?