Finally, from a morale point of view it acknowledges that they will get home and Harry's career will continue. Saying "Well, you're an ensign because that's what you were when we got lost" effectively says, "You're stuck." That's a morale killer.
This. Especially when it's not done uniformly or fairly. Tuvok gets a promotion. Tom gets one as well, for a year and a half of "exemplary" service. But Harry, despite six years of equally exemplary service, gets nothing.