What good is half a wing?
The question, "What good is half a wing?" is often used by creationists who severely misunderstand evolutionary theory. The question implies that fossil birds should be discovered with literal "half-wings" — i.e., a wing missing half of itself. This is a misunderstanding of how wings likely evolved.
According to the modern theory, evolution is not oriented toward a final goal but merely involves small changes that aid in the survival of individuals. In particular, evolution in the ancestor of modern birds was not working toward full-blown modern wings as we know them today. The arms of, say, small dinosaurs might have developed small proto-feathers in order to help them stay in the air longer when they jumped to avoid predators or to pursue prey. This wasn't yet a wing, but it ostensibly had some small benefit to the animal. Over millenia the feathers may have grown longer, the arms leaner and the muscles stronger. Today their ancestors, modern birds, do have wings as we know them.
The "half a wing" argument is related to the idea that evolution is like a tornado moving through a junkyard and assembling a fully-functional 747.