I disagree with blanket statements like this. Rainbow color maps aren't ideal for showing relative difference in values. But they are great if you want to be able to tell what the value (range) in a given location is. This is almost impossible with a 10-shade grey-scale legend or a height map.
Disclaimer: I use rainbow color maps all the time because I'm often interested in critical value areas of my plots.
So for me reading a Jet plot is basically impossible, they just doesn't make sense. Like, if a value is weak it is the brightest white, then if it gets even lower it gets blue. It peaks at green in the middle and then goes darker and darker as it gets stronger. Totally unintuitive.
Edit: Even worse is that how I see it depends strongly on what medium you use for the red and green channels. On some screens/paints red is basically black while others are almost fully green, depending on exactly where in the spectrum it is.