It's always good to have more girls, doesn't their combined level increase some of your stats? So the more the better? And more girls=more money. And like someone else said, they changed the system awhile back, so that every girl of a certain rarity costs the same. There is literally no detriment to collecting more girls, except saving kobans for extra chances at ones you really want. But for the free ones, go for it. There is only benefit, even if you don't level them anytime soon. Eventually your team of choice will be maxed, then you can slowly upgrade the rest of your collection and increase your revenue stream/stats.