Your assumption is twofold:
1.) It is useful to question it.
2.) The bias is due to ignorance.
That's not the case. Their bias is not due to ignorance but due to prejudice, and you cannot beat prejudice with logic because it is not founded in reason. If you question it, they'll just label you brainwashed and uneducated - look at the museum on medieval Tibet. People who are biased will say that all the evidence of Dalai's monks being slaveowners and torturers in there is fake, despite seeing the evidence right in their face.
The real way to combat prejudice is with assertiveness and strength.
Yes, I have the first assumption, but not the second assumption.
I can apply this to life in general, not just in politics. Obviously, biased rhetorics are not always the result of ignorance, especially when it involves power struggles. And in politics, it's mostly about the struggle for power. Various biased political groups, or lobbyists, are very well informed in fact. So no, I do not think they are ignorant.
However, I'm not targeting the people with this kind of bias, to change their bias through logic, etc. I still maintain that the first assumption is correct. To question and critique people's bias opinion
is useful because there are many other uninformed people who can be influenced by someone's bias opinions.
And tell me honestly, if you are an uninformed neutral person, what kind of arguments do you think are the most compelling, the type of article by that Michael Cole guy I posted or those one-liners by the fiddy and fenqing as commonly seen recently on PDF?
Like I said, this applies to life in general, not just in politics. I think you've mentioned before about defamation law. Why do we need them?
As for the people with bias opinion and agenda, I agree you cannot change them with logic and reason.