My reasoning for this was as follows:

It can be equally confusing for the user to try to do e.g. a calc on a field they are not displaying and find the placeholder is missing (your scenario) as it is to see multiple records from a repeat group in a list where none of the elements of the repeat group are shown.

To me it is conceptually clear that we are only retrieving by default the fields we are showing and we can easily make the tip for the Include List in Query describe the circumstances in which they should set it to Yes.