Here's another reason why rats shouldn't be used in experiments: They can recognize pain on each other's faces.
In a recent study, researchers took photographs of rats grimacing from an electric shock to their foot. They then took photos of the same rat in a neutral position. These images were scattered around a sort of maze, and other rats were given the chance to explore the apparatus.
What they found was that rats would spend less time in chambers with pictures of the pained rat than chambers with pictures of the neutral rat, indicating that the rodents could recognize pain in the face of other rats and avoided looking at them.