[Morthoron]

Well, the faceless Sauron of LotR is purposeful by Tolkien, as the 'Great Eye' description and the 'Mouth of Sauron' are indicative of the facelessness and impersonalized abstraction of the Dark Lord. This is perhaps Tolkien's manner of showing the diminution of the heroes of LotR in comparison to Sauron himself.

The heroes directly involved in the conflict, such as Aragorn and Gandalf (and even more so the Hobbits), are no longer on par with Sauron, or at least the egoistic Dark Lord feels he no longer needs to meet his foes face-to-face; whereas, there was a danger imminent in previous encounters (with Gil-Galad and Elendil, Pharazon, Huan and Finrod, for instance), that either precluded intermediaries from involvement or required direct intervention on Sauron's part.

In his megalomania (for I believe Sauron had become megalomaniacal, as opposed to over-confident or conceited, as those persons with megalomania also worry, mistrust and suffer paranoia), Sauron would not deign to meet in combat these 3rd Age has-beens and never-wases; instead, he imbues the WitchKing and Nazgul with powers necessary to marshall his troops, and he trots out the Mouth of Sauron to treat with sarcasm and disrespect the little lords of Gondor and their toy army at the Morannon.

By the end of the 3rd Age, Sauron has become a living symbol of incarnate evil, godlike in power and unapproachable. A symbol, not a personification.