The em-dash is of much more concern to copy editors and proofreaders in
publishing houses than to grammarians or English teachers.  Having worked as a
medical copy editor, I offer this example of an ad in a Philadelphia weekly
newspaper:
Non-Insulin
Dependent Diabetics

Participants for this Non-Insulin Dependent Diabetes medical research study
must:  etc.

This is an instance in which the em-dash, which is longer in length than a
hyphen, would clarify that the people wanted for the study are diabetics who are
not insulin dependent.  The longer dash separates the non from what it modifies,
in this case insulin dependent.  As it is, the words imply that those wanted are
diabetics dependent on non-insulin.

Writers don't use em-dashes; copy editors do.
Hope this helps, Connie