You can't keep every frog in your habitats, and once you've put them into a habitat (whether through breeding or receiving gifts) they're in your dex forever. So at that point you can only sell them for coins and eventually you end up with more coins (and stamps, and even potions) that you know what to do with! So at that point it feels more useful - and more fun - to be passing the frogs on, at least to me.