Well just for general morale. Promotion parties are fun and you have to find a way to motivate that crew somehow. I would make Harry a J.G. if for no other reason than to set him apart from his peers. (fellow Ops personnel if there are any) The same for Torres. Since Janeway was just picking out arbitrary provisional ranks for the Maquis people anyway, why didn't she just tell Torres to be a LT? That one didn't make a whole lot of sense. Maybe Janeway just didn't like Carey.