As I understand it, an expletive must be used when spelling out the truth in brutally honest, factual language would be too horribly painful or ugly. Use of the expletive allows the listener's feelings to be spared; the experience of hearing the expletive will be less damaging to his sensibilities. It will also protect the speaker from having to wash out his mouth with soap after stating the most unpleasant details of the reality of the situation.
Alternatively, the speaker may choose to say nothing at all, saving himself and his listener from the terrible impact of exposure to an expletive.
