I'm worried that Tony Adragna may have misconstrued what I intended by “rational ignorance” when he links to my blurb about Eugene Volokh from his post on Leon Kass. I shudder even now to mention them in the same breath. (Volokh good! Kass bad!)
Rational ignorance, in the sense Volokh was talking about, has to do with the opportunity costs of thinking. This is a big notion in voting theory. The democratic ideal is full participation by a fully informed citizenry. However, gathering sound information about candidates and policies is expensive, requiring a great deal of time and mental energy (and critical thinking skills that are also costly to acquire.) Given that the chance that any one voter's vote will decide the election is approximately zero, there is very little expected payoff in becoming informed. It is more rational to expend time and energy doing things that will have a payoff. Thus it is rational to remain ignorant of candidates and issues, and studies have shown that most eligible voters are indeed rational in this sense — they know next to nothing! (They might have a very nice lawn instead.)
At lunch, Volokh was using the notion to explain why citizens might be rational to consider existing policies to be well-considered, and thus biased to accept new policies that extend the principles of present policies. It's cognitively economical to defer to experts, and legislators seem (to the folk) to be experts, so the fact that something is a law creates a rational presumption in its favor, which may then extend to new but similar policies. And that's how (very crudely put) you get on a slippery slope. He's not saying this is a good thing; it's just what one might expect on an assumption of rationality.
Kass's “wisdom of repugnance” isn't about ignorance at all. He's saying that our feelings are sources of knowledge about ethical matters. You might say that Kass has a theory of rational passions — a theory that our visceral gut feelings are reliable guides to rational action. Now, I happen to think that this view is ignorant, but it's not about ignorance.
Probably I completely misunderstood what Tony was thinking, but it's a hoot to expound on rational ignorance anyway.