Guilt by Association is the "dirty bomb" of rhetorical techniques. Much as imitation nuclear weapons are built only by nations unable to figure out or afford the real ones, guilt by association is the reflexive refuge of people who aren't smart enough to think of a better, more subtle logical fallacy to use. It's cheap, easy, and plays directly into prejudices and stereotypes that pose as legitimate heuristics in the minds of the inattentive public.
Senator Joseph McCarthy, 1951:
"Are you now or have you ever been a member of the Communist Party of the United States?"
Right-wing shill Howard Kurtz, 2007:
"But you bash the Bush administration so often that you have become a hero to some on the left. I mean, just in this book, in the first few pages, you talk about Bush and company harming America. You call the president deluded and you speak favorably of impeachment. Do you consider yourself a left-winger?"
The times and names change, but the rhetorical tactic is identical – admit that you are a member of some group or category that will allow us to discredit you and question all of your motives. If the media reports something that contradicts what you believe, cite "liberal bias" as evidence that it must be a lie. Most people, of course, don't even know who are the reporters behind the hundreds of daily newspaper/TV items credited to "Associated Press" or "Gannett News Service." But if he or she is a reporter, then obviously he or she is a liberal. All reporters are not merely liberals, they are lying, deceptive liberals out to distort the truth.
Given what a transparently lame argument it is, Guilt by Association is rarely used except by people who are unable to defend their position any other way. Or by really lousy writers. Like James Taranto:
The most telling moment in last night's [State of the Union] speech came after the president noted that "key provisions of the Patriot Act are set to expire next year." In response, notes the New York Times, "some critics in Congress applauded enthusiastically." If Osama bin Laden watched the speech, one imagines him applauding too.
If Osama bin Laden likes something and you also like it, you are his comrade and supporter. Just like how Hitler liked gardening and I like gardening, which confirms that I am a rabid Nazi. The ACLU thinks terrorists should be tried with due process, so the ACLU are terrorists. Your professor does not think Reagan was the greatest president of all time, so he is an ivory-tower liberal trying to brainwash you. What excellent, logical reasoning.
I just feel bad for people who use this argument. It usually indicates desperation or, more often, a stunningly superficial capacity for understanding any substantive issue. Sure, we could talk about why the Patriot Act is controversial…but John Doe realizes that if he just labels everyone a Patriot or a Traitor he can get to Trick My Truck that much faster.