I thought middle names were mostly so both sets of grandparents could claim the kid was named for someone in their family.
I know quite a few people who have a middle name that is actually just a single letter, such as G.
I know quite a few people, including my brother, who are always called by their middle name but never by their first.
All of my relatives from Florida are called by both their first and middle names all the time, as in Mary Francis, Edna Oliver, Bobby Jo, and Billy Joe.
And my son has two middle names, one from my husband’s side of the family and one from his birth parents. His first name is actually from both my husband’s and my sides.