If you need gear and the GC seals help you get it, take the gear. You earn seals faster at higher level so you can always catch up if needed.

Otherwise, yes. Having higher ranks lets you do expert delivery, where you can turn in unwanted dungeon drop gear for seals, which earns you tons of seals. You can buy a lot of things that have value from your GC (like ventures, crafting mats, cordials, etc), so that's a useful thing to have. 2nd Lieutenant also unlocks squadrons, which lets you get to 1st Lieutenant and can earn you extra rewards.