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 to the age of reproduction. 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 millennia 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.
What good is half an eye?
A closely-related question is, "what good is half an eye?" But we all know people who are nearsighted or farsighted, and who get along quite well. Even without glasses, it is better to be nearsighted than to have cataracts, which in turn is better than being blind. Half an eye is 1% better than 49% of an eye.
Another way to look at it is to ask what constitutes 100% of an eye, the answer to which can shed light on what 50% of an eye is, and therefore what good it is.
The nerves that carry visual signals from the retina to the brain are wired so that they go inside the eye. Where they exit the eye, they crowd out light receptors, resulting in a blind spot that the brain must fill in essentially with educated guesswork. Squid and octopus eyes are not wired this way, and therefore do not have a blind spot. Thus, humans cannot be said to have 100% of an eye.
Human eyes have a fovea, an area densely packed with light receptors, which we use for distinguishing colors and for resolving fine detail (this is why we can't read a book out of the corner of our eye: there aren't enough receptors to make out the shapes of the letters). But some birds of prey, such has hawks, have two foveas, which allows them to resolve detail in two areas at once. Humans, therefore, have less than 100% of an eye.
Chameleons have independently-targetable eyes: each eye can look at, and focus on, a separate object. They can estimate the distance to each object by the way the lens focuses on that object. Humans can only focus on one object at once, and therefore have less than 100% of an eye.
Most people have three types of cone cells, and therefore see three primary colors. But some rare women have four types of cones, and therefore can distinguish colors that other people cannot. Thus, most people have at most 75% of an eye.
By putting these and similar criteria together to establish what a hypothetical ideal eye might look like, we see that most people have far less than 100% of an eye. And yet it works well enough for us to see.