[Dimturiel]

Perhaps the Mouth of Sauron used his master's name at the Black Gate to play with his enemies, to sort of daunt them. I mean, if all went his way - and, at that point, I don't think he had too many reasons to believe it would not - he would have taken Saruman's place at Isengard and he had to secure his position to his enemies, to make sure they would really never bother him again. He wanted to prove to Gandalf and the rest that he was not just a slave of Sauron's - although Gandalf does call him one at the end of their discussion, thus showing that the trick did not actually work. It was like he was saying to them: I am that deep in Sauron's favour, that close to him that I can actually afford to use this name, a thing he does not permit the rest of his servants to do. Therefore, I am to be feared almost just as much as Sauron, and you'd better fear me, for your own good. I mean, what better way to make sure no one would dare to openly challange his position in Orthanc than by proving that he was the second most powerful being in Middle-earth apart from Sauron and that Sauron sort of acknowledged that by allowing his ambassador liberties he would not normally have allowed to his other - less trustworthy and less important, it is implied - servants?