If the backbone - spinal cord is made of a long bone rather than a series of small bones (segments) then we will have difficulty in movements.
There will be problem in bending forward, backward and sideways with flexibility. The shape of the back of human body is due to the shape of the spinal cord. The stress withstanding is more with the segmental structure.
It is a better design too of the control of the muscles through various segments of the spinal cord. The different segments are responsible for mobility, flexibility of muscles in different parts like: the hand, elbow, wrist, thighs, legs etc.
With a single bone structure the design may not be so good.