You are certainly correct that civility and dialog are essential to achieving any political result in a democracy ...
DL's statement hits on one of the most long standing debates in Western history: What is effective discourse? If you hold that effective discourse is that which progresses your cause, then the sentence is false. The ridicule that we see flying from the mouths of Ann Coulter, Steve Colbert or Al Franken is extremely effective. So to is the hate speech. Plato and Aristotle were both against democracy because they saw that uncivil discourse was better at moving the sentiment of the masses than civil deliberation.
(NOTE: open ridicule often backfires. Generally subtle ridicule such as the use of praise words and snarl words as practiced by Chomsky, politically correct universities or by the main stream media are more effective than open ridicule).
My observation is that the classical liberal tradition held a different view. This view seems to hold that effective discourse is discourse that leads to optimal results. In the classical liberal tradition, DL's statement is true. Civil discourse allows you to bring the debate from the subliminal to an overt level were you can better see cause and effects, and make better decisions.
Negative discourse is effective in either pushing a party or cause. Eventually, however, negative discourse leads to a fractured society or a fractured government that is incapable of doing anything.
BTW, some political humor is needed in the world. Come to think of it. Political leaders tend to be buffoons, and need to be knocked off their high horses on a routine basis. There needs to be at least one political cartoon with each editorial page. In the long run, to thrive as a society we need to support civility in discourse.