It is not required for just a tub, as long as you do not install any sort of hand held sprayer with it. The idea is that you can be in the shower, and a sudden change of temp can hit you without warning. You are a "
captive audience" to that event! The tub, it fills over a long time, and even if you are in it, if there is a sudden change, you have time to reach up and turn off the hot or whatever.