Violent rhetoric and violent behavior
Much of the debate since the Tucson shootings has focused on what seems like the wrong question*: was there a direct influence from the extremist rhetoric of the past two years to the violent actions of this particular assailant? Sometimes the answer to this kind of question is "yes" -- Timothy McVeigh was directly inspired by the violent ideas and passions associated with the right-wing militia movement. But the harder question is that of indirect and diffused influence: is it possible for a pattern of virulent media communications to create a culture of violent attitudes that leads through indirect mechanisms to political violence directed against individuals and institutions?
Ding! The answer?
It doesn't appear that direct psychological research has yet been done on this question. But there is a related question that has been very extensively studied, and that is the effect of dramatized television violence on children's propensity for aggression. It appears that there is fairly strong evidence in the social psychology and developmental psychology literatures for a causal link between exposure to television violence in children and increased aggression. Here is a paper in Developmental Psychology by L. Rowell Huesmann, Jessica Moise-Titus, Cheryl-Lynn Podolski, and Leonard Eron (link), where the authors find a significant link between childhood exposure to TV violence and adult aggression.
So these seem to be fairly strong empirical reasons for being very concerned about the inflammatory language that has become increasingly common in political discourse and media rants. The issue isn't simply the value of political civility; it is the very real possibility that extremist rants can influence a small number of listeners to be more prone to engage in acts of political violence, and even people who aren't listeners can be influenced by those who are.
NOTE * Actually, if your (trollish) goal is to defend the purveyors of violent rhetoric, this is exactly the right question.