The scions had to split up and defend multiple areas from Lunar Primals. It made sense that the WoL couldn't just go around fighting these primals one by one, due to the severe impending threat, and that the other Scions would have to fight these primals themselves. Given that these are special lunar variations of old primals, I would have been disappointed if they were just handily dispatched in a short cutscene, or worse, offscreen, and not have the opportunity to fight them myself. In this regard, I fully appreciate body swap duties, in that they let me experience the fights myself, while also giving at least one character a turn in the limelight