"According to a study in Respected Peer-Reviewed Journal, use of Type A widgets is effective in lowering the risk of Bad Stuff."
"Respected Peer-Reviewed Journal? It's probably biased."
Rhetoric is the art and skill of argumentation, analogous in some ways to martial arts in personal combat. Skill at rhetoric enables one to win arguments more often, and to defend one's positions more effectively, and we can think of winning an argument as being analogous to winning a fight (though I will argue later that this is the wrong way to look at it.) As with martial arts, there are some flashy moves that amateurs like to use, but in reality are almost never tactically sound. The accusation of bias is one of them.
It's easy to see why this would be so tempting to use. It looks, at first glance, like a devastating offensive maneuver. With one simple claim, you seem to disarm your opponent completely, depriving her of all the evidence upon which her opinion rests. Bam! Game over! What choice does she have now, but to accept your position as correct?
But in fact it's a clumsy and amateurish move that does more harm to your own credibility than that of the opponent. You cannot raise the issue of bias without calling your own objectivity into question. And when that happens, you're at a disadvantage, because you've already demonstrated an eagerness to discount unfavourable evidence the instant it looks like it might be unfavourable.
Worse, you surrender any realistic hope of convincing your opponent when you play the bias card, because you undermine the very basis upon which convincing happens. If evidence and reason can be arbitrarily dismissed as "biased" because it supports one side or the other, then what reason is there for your opponent to accept any evidence or reason whatsoever? At best, the bias card is a scorched-earth defensive weapon; it allows you to avoid being convinced by your opponent, but that's all it does.
However, I want to argue that it's even worse than that, because a true rhetorical warrior's objective is paradoxically not to win arguments, but to lose them, and by taking a step that makes it impossible to lose an argument, you make it impossible to succeed at rhetoric.
The true rhetorical warrior is seeks not to convince his opponent, but rather, seeks to be convinced by her. True victory is to be persuaded, genuinely and honestly, to adopt a new opinion. It is no good conceding prematurely, pretending to be convinced by weak arguments or trickery. He must be sincerely convinced, satisfied by his opponent's logic and evidence, that he ought to adopt her claims. To that end, he presents his objections not in an attempt to refute her, but to help her understand the obstacles to convincing him in the hopes that she may overcome them.
To be persuaded is a victory, because it enables one to improve one's understanding of the world by abandoning an understanding that is demonstrably flawed. "Winning" an argument, in contrast, profits one little, however gratifying it might be to one's vanity. The rhetorical warrior who understands this, then, would never try to play the bias card, because there is nothing to be gained by playing it.