To call them anything but Nazgūl is euphemistic. Nazgūl is the most evil and most applicable term for them. Anyone wishing in a certain situation or in general to make them appear utterly evil does not hesitate to use the word. Only someone who has an enormous fear of them, like Gollum or the hobbits, will not dare to speak their proper name. Even the word Mordor has dark connotations, and is not used lightly, at least not in the Shire.
So the reason that the word Nazgūl is used is the same reason that Gandalf chose to use the Black Speech at Rivendell - for effect.
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But Gwindor answered: 'The doom lies in yourself, not in your name'.
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