Actually, that's an example of why it would be useful. If one profile in a merge had duplicates, they would add to the count. You could tell at a glance that something was off.
Of course, a correct count could still be off because of duplication! Still, I would find a headcount to be a useful quick-glance tool.
I don't think the headcount should appear on the main page of the profile, just on the edit page where the family members are listed (on the right side of the "Edit" page under "Edit Family").