There are places in Moodle where this would be beneficial.

1. Create a course.
2. Enrol a user as a student.
3. Add a feedback activity with 'Record user names' set to "User's name will be logged and shown with answers".
4. Click on the feedback activity.
5. Click on the tab 'Show non-respondents'.
6. Notice that the 'Send' button is not a primary button, which is fine, until you enter text, when entering text and the form is no longer empty it should become primary.

I am sure there are other places in Moodle where this would also apply