According to Mother Jones:
In a recent study, a team of researchers from the George Mason University Center for Climate Change Communication and several other institutions employed a survey of 1,183 Americans to get at the negative consequences of vituperative online comments for the public understanding of science. Participants were asked to read a blog post containing a balanced discussion of the risks and benefits of nanotechnology (which is already all around us and supports a $91 billion US industry). The text of the post was the same for all participants, but the tone of the comments varied. Sometimes, they were “civil”—e.g., no name calling or flaming. But sometimes they were more like this: “If you don’t see the benefits of using nanotechnology in these products, you’re an idiot.”
The researchers were trying to find out what effect exposure to such rudeness had on public perceptions of nanotech risks. They found that it wasn’t a good one. Rather, it polarized the audience: Those who already thought nanorisks were low tended to become more sure of themselves when exposed to name-calling, while those who thought nanorisks are high were more likely to move in their own favored direction. In other words, it appeared that pushing people’s emotional buttons, through derogatory comments, made them double down on their preexisting beliefs.
So basically, uncivil comments hurt discussion. They polarize the audience and make it harder for people to change their minds. Libby Anne has a very good post about the consequences of this troll-like behavior.