My niece's middle name is K
Not Kay. Not K. Just: K
She's 17 now. Cute. But just imagine as she gets older, when she's filing out forms that require a full middle name.
In the same way, Truman's middle name was just S
I don't know what forms legally require a full middle name, maybe some do; nothing I've ever filled out demanded it, but I don't have one. "You'll get married", my Mother said, "and your last name will change anyway", operating on the premise that middle names are needed only to distinguish between Jane (Ann) Doe and Jane (Mary) Doe.
I'll tell you what else Mom did to me: she named me X, supposedly after my Grandmother X, except my name is not exactly Grandmother's name -- a sort of derivative -- and then she nicknamed me a third name so no one would foreshorten my name into an other, unattractive nickname. Got that? The only saving grace is none of three/four names is uncommon or weird. It's Mom that's weird.
What people name their kids today, though, is headshakingly, dumbfoundingly ignorant.