Christof Koch, in his The Quest for Consciousness, claims that dual strategy beings can outcompete zombies. Yes parts of the brain are designed for rapid, single purpose use, as you might find in a zombie. But other more integrative and judgmental parts require more powerful central processing units, namely your conscious mind. In his view consciousness is not just an epiphenomenal feeling, as in much analytical philosophy, but rather it is a functional set of qualia. In other words, consciousness helps you interpret "meaning" and thus use information about the natural world more effectively. Consciousness allows you to summarize the present state of the world in abbreviated fashion and to make plans on that basis. Consciousness is a sometimes-slow but always flexible strategy.
Here is one good summary of the argument; read this excerpt:
Consider the following situation: You see an outstretched hand, but instead of shaking it immediately, which instinct would dictate, you are required to close your eyes and wait several seconds before doing so. Koch and Crick suspect that without a short-term memory, a zombie could not do this task, or any other in which an artificial delay was imposed between “an input and the associated motor output.” Absence, like presence, has a neurological signature, and Koch imagines a kind of “conscious-ometer” that could be used to measure who and what is consciously aware.
Note also that efficient zombie-like behavior often requires conscious learning in the first place. Isaac Stern might best play the violin by "letting go," but he first needed many hours of conscious practice to reach this state. So consciousness and zombie behavior are often complements rather than substitutes.
If you are interested in these issues, this book is the place to start. Try also this skeptical response.