One legitimate use I have seen for using -- as a surname is when a character’s name doesn’t fit the two name schema that we use in much of the real world. One thought I had to remedy this is to have how a Name-Surname pair is displayed on a profile be variable. I’m gonna call out @Tlancaalek here, as he’s stated in the past that his name is structured in a way that Lek is like his given name and Tlancaa is like his surname.
What if, as a part of the profile, a formatting string is supplied, defaulting to %N %S where %N is a placeholder for the name field, capitalized, and %S is a placeholder for the surname filed, capitalized. Then, if Lek wanted to, he could change his name to name: Lek, Surname: Tlancaa with the formatting field be %S%n, which would reformat his full name to appear as Tlancaalek where full names are used, but simply Lek where only the name field is used, thus eliminating his need for a blank surname field.
Other places I could see this useful are on Far East-styled names, like in Japan and other cultures, where the surname is ordered first, before the given name. I could see a future realm build for an anime focused audience or a Japanese, Chinese, Korean or Vietnamese speaking audience, where the default format string is changed to %S %N instead of what most western-language oriented realms would have.