It is trying to discourage the use of the nested Elm architecture pattern Nested TEA - Elm Patterns. Commonly Html.map is used in this pattern. This pattern is useful but is best to leave it for when it is really needed. It brings a good amount of complexity and boilerplate, so it better to find alternative solutions first.
If Html.map is useful for you without the whole Nested TEA pattern then there is not reason not to use it.