Animation is simply not just tracing of a image that was already created. The amount of time it takes to develop a fluid, believable scene can take a week or two (if not more in some cases). By simply reusing the animation from previous films, it takes out the entire process of figuring out whether the scene works or not...they already know it works, because it was good enough to have been placed in a prior film.
For those who don't know....
Keep in mind, Disney works with key-framing and very, very little straight ahead animation. The animator first has to act out the entire scene, thumbnail how they think it's going to work and then make their key frames (extreme poses). If it doesn't work or it isn't what they thought it would be or the director isn't crazy about it....then they are back to square one and they have to repeat the creation process. This is very, very time consuming, using old animation actually removes this entire step, thus saving money and time.
Once the animation is okay'd by the director, then it's passed to the in-betweeners. These artists fill in the necessary drawings missing from the key-frames. The entire creative process in making the animation at this point is over....these guys just have to fill in what is already in front of them. Lead animators leave them notes and sometimes even a sheet of thumbnails so the inbetweener knows exactly what happening.
Next....clean up....then....scanning....color. The whole cel process is very dead concept at Disney, you can usually find it at small animation companies that do a lot of television commercial work. Even the small one that I worked at for four years has turned completely digital (bummer). It, like copying animation, is much cheaper and faster.
