I can safely say that I am not personally acquainted with any illeists. I'm not sure whether this is down to good luck or whether I have had the good sense to purge anyone who refers to themselves in the third person from my life. What an annoying habit it is. Hearing someone use "he" to talk about himself or, worse still, use his own full name for this purpose is like fingernails on a blackboard for me.
It seems to be predominantly a male habit. There are no women on this list of 11 Famous Illeists which includes Bob Dole, Elmo, Julius Caesar, Salvador Dali and Charles de Gaulle. Years ago I used to have regular meetings with an illeist. I don't think I ever really engaged with what he said; I was too busy wondering why he kept using his full name instead of "I".
I have to confess I briefly dabbled in illeism when my daughter was little. Yet phrases such as "Give the spoon to Mummy" and "Mummy loves you" are surely forgivable when your target audience is a preschooler. Age-appropriate illeism aside, what drives an adult to speak this way to fellow grown-ups? Just as importantly, how can they not know that most people disapprove of the practice?
Esquire describes it as a "Major human flaw" and suggests that it's about control, a power play - that it's how "you indicate that the topic is not open for debate. You are speaking about facts that just so happen to include you."
Illeism can sometimes be used to denote subjugation, servitude or inferiority - as in "Your servant awaits your orders" and "This slave needs to be punished". In the Harry Potter series, Dobby the house-elf speaks of himself in the third person. Used in the military - as in "This recruit is reporting for duty, Sir" - its purpose might be to diminish the speaker's sense of himself as an individual.