Ilya Somin at Volokh Conspiracy methodically answers my questions about extremer extremists.  My original suggested response umbrellas:

1. Public relations. Views more extreme than your own are
counter-productive because they alienate the moderates you need to
convince to get better results.

2. Transition costs. While you agree with the extremer
extremists about the ultimate goal, they underrate the transition costs
of getting from here to there….

3. Latent pluralism. Despite your often one-sided rhetoric
and disdain for the “other side(s),” they actually make some valid
points; they just overstate them. Thus, even if you habitually dismiss
the view that statist policies give bad incentives, you might ultimately
agree that your policies would provide disturbingly bad incentives if
they were pushed further than you advocate. Picture a socialist who
opposes a 100% marginal tax rate….

4. Papered-over fundamental differences. Even if you
psychologically and sociologically identify with your extremer
extremists, you don’t philosophically identify with them. They’re just
fellow travelers who fail to grasp the principles that really count….

Somin’s response:

Bryan’s point 1 isn’t really a reason to reject the more extreme
view. At most, it’s a justification for not revealing that you hold that
position, in order to avoid alienating moderates. A genuine “extremer
extremist” can still choose to seem more moderate than he really is for
public relations reasons. In any event, I don’t soft-pedal the substance
of my views on issues I regularly write about for the sake of
attracting moderates, though I am very conscious of this issue when it
comes to questions of style. I might act differently if I were running
for public office or gunning for a judgeship. But fortunately I’m not.

Point 2 is potentially significant. There are various government
programs whose creation I consider to be unjustified that I would not
abolish immediately, because of reliance interests. In most such cases,
however, I would still want to abolish them gradually rather than leave them in place permanently. So this is not really a big area of disagreement between me and more extreme libertarians.

The fourth point is a bigger issue for me. Many of the libertarians
who are more extreme than I am believe in absolute property rights,
whereas I do not.
I think utilitarian considerations matter also, and individual rights
(including property rights) can sometimes legitimately be sacrificed if
there is a large enough utilitarian benefit. However, some libertarians
who are more extreme than I am actually hold very similar fundamental
values. Economist David Friedman and Bryan Caplan himself are good
examples. Both of them also reject absolute rights and are partial

The really big factor for me is ultimately point 3, “latent pluralism.” There are a few market failures (mostly certain public goods problems)
that I think private sector institutions can’t handle, while government
has at least a reasonable chance of doing better. I think liberals and
conservatives (to say nothing of socialists) greatly overstate the
frequency of such examples. But I believe they are correct about a small
but important set of cases. I’m familiar with the more extreme
libertarian and anarchist literature arguing otherwise, some of which
makes excellent points. But I don’t find it fully convincing.

Read the whole thing.