When a postmodernist examines an article or opinion the first question he asks is, "Who wrote this? Whose opinion is it?" Following this, the postmodernist says to himself, "What position are they coming from? Why do they take this position?"
"Oh," s/he says to her/himself, "This person has a vested interest."
This then becomes a reason for being suspicious of the person's reasoning. The vested interest, prior position (Christian, white male, being middle class, working for the oil industry once, etc etc) delegitimises that person's argument, before they have even said anything.
When people believed that it was possible to argue about things rationally in a free society, this would have been called an 'ad hominem attack,' one of the standard logical fallacies. 'Ad hominem', 'against the man' in latin, means to argue that a person's position is wrong because of the person. To be logical one must argue against the logic of what they are saying, rather than who they are or what group they come from.
What this kind of postmodernist rhetoric does (in the name of truth) is makes democracy impossible: if you can't argue a point from logic agreeing that the other person might be just as right as you are, if you are saying, "this person cannot be logical because they are a member of a particular social group" - then you may as well stand there singing 'la la la la' with your hands in your ears as have a conversation where you try to get at truth, or an argument where you try to convince others (i.e. democracy) if you have already decided that whatever they say is influenced by their 'vested interest' and is therefore not worth listening to.