OK, so get this, I saw a couple of youtube videos on the best Raspberry pi projects, and a couple of them were robots right? But all the extra effort went into making the raspberry pi do what Arduino is already optimized to do, which is robots. Raspberry pi is optimized for making things like portable stereos, homebrew playstation portables, and mini AI brains for your robots. They are not so good at moving arms and legs or sensing temperature and optical changes in the atmosphere while running pulse width modulation variables.
Arduino is good at simulating the nerves along the spinal cord, through your finger tips, optic nerves, ears, and controlling your muscles. Raspberry pi is good for making your robot knowledgeable in calculus simulations, storing medical procedures, reading aloud in a robotic voice the works of Shakespeare, or playing the latest mp3 sound track while it sends dance commands to the arduino shield, so that the arduino can order the servos to make your robot do the cha cha.