I am the Mouth of Sauron.

[Lalwendë]

Rather than being anything to do with rank, I think the Mouth of Sauron was listened to by the hosts of the West because it was a matter of honour to treat a messenger with respect, no matter who he was. Tolkien places Aragorn, Gandalf etc in the position of honour by showing that these people at least can show respect to a messenger. I wonder if Sauron would have done the same? I think not. And that's one aspect of the films which annoyed me as film Aragorn displays mere anger by chopping off his head, whereas he ought to have been shown simply letting the messenger go or else just detaining him.

It makes sense to me that Sauron would have staff (servants? minions?) from other races, as this would be of considerable help in bringing peoples of different races around to his way of thinking; those he hoped to subdue might be more readily won round if 'one of their own' was dealing with them, gaining their sympathies etc.