Coffee and tea both have a history of being used in desserts solely for their flavor, and using the tea leaves or coffee grounds once for tea/coffee probably doesn't remove so much flavor from it that it renders it inedible. If you drink coffee or tea it's an easy enough experiment to perform yourself at home the next time you bake something. Even if the end result is "adds no flavor enhancement at all" it might still be worth doing just for the added shelf life, the same way that preparing bread with the tangzhong technique (pre-cooking some of the flour with water on the stove before incorporating it into the rest of the dough) doesn't alter the flavor but enhances the texture and shelf life of the bread.