I agree with you here. But I have a little problem understanding what you mean. One of the main attractions to libertarianism is the understanding that no one has all the answers. If I'm correct here, then their are no such libertarians as you claim. Maybe you imagined them.
...too much reference to one's opponents as diseased, depraved, or mentally deficient doesn't win converts.
On this I agree. There are many former democrats who today stand against the Democratic Party. Many of them had been converted. Being wrong does not make one "diseased, depraved, or mentally deficient."
Some irony or skepticism is also valuable -- if not with regard to one's beliefs, then at least with respect to one's ability to live up to them or express them in art: Rand wasn't one of the great novelists herself, and plenty of the writers she admired probably wouldn't be impressed by her work, an indication that she may not have had all the answers in aesthetics either.
There are also the Rockwellites, who go through history giving A's or F's to figures from the past, based on their economic views. Now of course, we do have to judge past leaders and thinkers, and it may be that tariffs or regulation are inherently bad and always liable to have negative consequences. But things looked different to people at different times.
The options that we have now weren't always available. One can't simply step into the past from an economics classroom, and say what should be done. People in the past lived their way through experiences that taught them different lessons from what Rockwell or Rothbard teach. Very often, one made a choice in dark uncertainty between options that were all bad.
We also don't have the evidence about what would happen if an alternative course had been pursued -- if we'd never had protective tariffs, or consumer and environmental protection agencies, or social security -- and people disagree about the possible consequences of such an alternative course. What we'd need is a model of what would happen if the Rockwellites got their way on everything. We don't have that, and it's very possible that such a world would have just as much wrong with it as our own.
Writing history usually requires more modesty and skepticism than the coach Rockwell and his armchair quarterbacks usually display. The way that historians like Schlesinger and Hofstadter or ideologues of the left like Chomsky and Zinn always claim to know what should have been done is embarassing and unattractive. When one sees that sort of cockiness coming from the right it's equally off-putting. In theory, libertarians do believe that no one has all the answers, but in practice it's not always the case.
It's more a question of manner than of matter. One can certainly say that this painter or that policy was disastrous. Sometimes a commentator has to say that. But it's the refusal to grant the other side its due and to admit that one may be wrong about specific works or events or situations that galls people. The conviction that one always has a moral ace up one's sleeve or the revealed truth in one's vest pocket often leaves a bitter taste in other people's mouths. My guess is that Tracy Ryan thinks that "you can't think rationally about economics" is the successful end of an argument. It looks more like a beginning -- and not a very promising one at that -- if it's not simply avoiding an opportunity to win people over and simply preaching to the converted.