I hear the criticism that shrill liberals (or even shriller conservatives) do nothing but preach to the converted, hence they provide no useful function:
Malefactors of Megawealth, by David Kennedy, NY Times: ...Like the rants of Rush Limbaugh or the films of Michael Moore, Krugman’s shrill polemic may hearten the faithful, but it will do little to persuade the unconvinced or to advance the national discussion of the important issues it addresses. It may even deepen the very partisan divide he denounces.
I think this is wrong, and not just because of the silly equivalence drawn between Limbaugh and Krugman. First, I don't buy that these voices cannot reach beyond the confines of the converted. Given the responses I've seen to Krugman and others, many of these voices penetrate the other side of the political fence quite well, generate responses, and in doing so help to advance national discussion on these issues. I have no doubt, for example, that Krugman has helped to promote national discussion on a variety of issues from health care to inequality to trade policy to Iraq.
But suppose it is true - suppose one does nothing but preach to the converted - does that mean there is nothing useful happening in the exchange?
What those with a voice can do is give people the arguments they need to rebut the other side's arguments in the daily exchanges at the water cooler at work, at dinner parties, at family gatherings, and so on. Whenever political arguments come up in the course of conversation they will have heard both the arguments and the counterarguments and they will be much better able to defend their views (and much less likely to be swayed by misleading messages from the other side). Echoing good arguments on various sites can reinforce this effect even if it is mostly talking to people with similar overall views. Thus, even if it's true the someone like Krugman sways nobody directly (a proposition I doubt), there is still an indirect effect where the arguments that he gives to others can be used to win the smaller day to day discussions between people, the discussions that impact the marginal undecided voter.
That's also why I don't mind having (tolerable and rational) conservatives in comments. They make their arguments, then we hear the rebuttal, excellent rebuttal in most cases. The arguments are out there whether I allow them in comments or not, so the important part is to generate effective counterargument that can be used more generally as people interact with others and these arguments come up in the course of the discussion. Winning the little battles in social discussions is important, and preaching to the converted is a key step in enabling people to have the persuasive arguments they need to be effective in arguing for their ideas.