On the surface, the logic behind using a rainbow color scale makes sense: the more colors there are, the easier you would expect it to be to see detail in a huge range of data. However, when perceptual issues are taken into account, rainbow color schemes are one of the worst ways to represent continuous data. There are five main reasons for that:
The image below shows simulated views of what someone with Protanopia would see for two different color scales. Clearly, the diverging color scale is better for Protanopes because there are no repeat colors.