I use those sorts of things as duck food. I'm sure you could also make veggie broth with them plus other extra veggie parts (like ends of onions, etc.) Sauerkraut with the cabbage leaves?
Members of the cabbage family usually are not added to veggie broths because of their strong taste. I might add a bit, depending on what the broth will eventually be used for, but not much. The chickens get the leaves from my kitchen now, but I agree with Drakemaiden, it isn't wasted if it is composted.