For MOST profiles, the Display Name should *not* be used so that the user name preferences can "be effective". Display Name should only be used in a few instances:
-- living/modern era "famous" people who where known by a different name; often actor/actress 'stage names'. e.g. Madonna, Elvis, ....
-- medieval & earlier era, particularly in Europe, where there was often only one name used and it is necessary to "disambiguate" names.
-- Royalty and Titles of any era.
Sure, those cases cover a lot of "well-known" and "oft-visited" and "commonly shared ancestors", but when compared to the 100+ million profiles ... it is still the recommendation that Display Name NOT be used for most profiles.
For 'immediate family', some people like to use nicknames, but the AKA field is usually a better place for that, unless the person used that a 'nickname' as their signature most of the time.