Good point. The typical scenario I'm referring to goes like this: 1980's house roughing. You have two bedrooms back to back about 60 feet from the panel. You want to run separate 15 amp circuits to each. You can drag two
14/2 cables, one per room, or drag one 14/3. If you bring the multiwire, you'd bring it into an outlet box on a common wall to both rooms and split one circuit for each room at that point. In the box where you brought the two circuit cable, you have an outlet. That outlet and only that outlet cannot have the neutrals dependent upon the receptacle, and must be pigtailed. Once you've exited that box, each circuit are just two wire single circuits, and all neutrals can be dependent upon the receptacles