There's no ethical obligation to be "honest" in argumentation in this sense. Arguments exist in the abstract; people are just argument delivery devices. If there exists an argument that shows that my philosophy is inconsistent so that I have to adjust the philosophy or change a position or even consciously choose to live with an inconsistency, I should deal with that argument, regardless of whether the guy presenting the argument is self-serving or a creep or Hitler.I think that this is right, but I think it's so obviously right that it barely needed to be said. Consider: the following form of argument is utterly uncontroversial:
Suppose A. [Reasoning from A to B] Therefore, if A then B.The supposition there is not an affirmation. It is a 'pretend this is true, and we'll see where it would take us'. So if I were to argue, for instance, that the principles of Christianity implied that we should legalize homosexual marriage, the fact that I don't believe the principles of Christianity in no way impugns my argument. In particular, "you don't even believe the principles of Christianity" is nothing at all like a rebuttal of my argument, and the Christian who opposes homosexual marriage still has an argument for his inconsistency staring him in the face. To suggest otherwise is to tell me that I have no way, even in theory, to offer an argument to someone who disagrees with me.