Hello dude. if its about any kind of advantage, no, getting new girls is really completelly useless. that said, harem girls are just trophies. being useless objects is quite natural.
back in the day, upgrading girls was much cheaper, and they produced more money. but this all changed and its obvious now that devs consider having harem with maxed girls to be end game content, so they made it as hard and tedious as they think they can get away with. have fun!