One objection I often hear raised against thick libertarianism is that bundling libertarianism together with other values and commitments is dangerous to the cause of liberty, because a) it tends to alienate potential supporters of libertarianism who don’t share the additional values, and/or b) libertarians whose support for liberty is conjoined with an equally strong support for other values face a greater risk of being tempted away from their libertarian commitments by the possibility of using state power to promote the other values.
The first thing to point out about this objection is that it is not strictly correct to call it an objection to thick libertarianism. On the contrary, this objection is itself a version of thick libertarianism (we might call it “anti-thickness thickness”). After all, its objection to bundling libertarianism with other values is that such bundling makes the achievement and/or maintenance of a libertarian society more difficult – and that is precisely a thicklib kind of concern, specifically a form of what Charles formerly called “instrumental thickness” and now calls “strategic thickness.” After all, the non-aggression principle by itself doesn’t call on anybody to promote libertarianism; so as soon as one starts arguing that libertarians qua libertarians should reject X (where X is not by itself inconsistent with the non-aggression principle) on the grounds that X makes it harder to promote libertarianism, one has moved from thin to thick libertarianism. All that one can consistently do in defense of thin libertarianism is argue that libertarians need not bundle liberty with other values; to argue, more strongly, that they should not is to turn thicklib by bundling libertarianism with a commitment to no-bundles-but-this-one.
The fact that this purported objection to thick libertarianism turns out to express thicklib concerns itself is not a decisive reply, however; for the objector can simply redescribe the objection as directed against forms of thick libertarianism other than itself. Still, the skinnylibber who brings this objection has admitted the relevance of thicklib concerns, so we have our foot in the door.
Turning to the specific objections: it’s doubtless true that thick libertarianism alienates some potential pro-liberty folks who don’t like the extra values in the bundle; but it’s equally true that thin libertarianism alienates some potential pro-liberty folks who have trouble feeling the attraction of libertarianism until they see how it integrates with their other concerns. One simply has to weigh these considerations against one another, and against other thick concerns (not just strategic but also application, grounds, and consequence), to see how it all sorts out.
As for the worry that devotees of Liberty-plus-X might be tempted to violate liberty in order to promote X, well, so they might – but a commitment to X, if it’s the right X, might well help to guard against certain sorts of temptation to sacrifice liberty. (See, e.g., Rothbard’s argument that a rejection of utilitarianism makes one’s commitment to liberty more reliable.) If a commitment to X instead undermines one’s commitment to liberty, then either one has chosen the wrong X – one that is not properly bundled with liberty (and so one that is itself condemned on thickness grounds) – or else one has misunderstood the relation between liberty and X. After all, thicklib values are supposed to be ones that have a natural fit with liberty, values that it would be unreasonable or counterproductive to pursue by nonlibertarian means.