Rothbard took liberty – including liberty of association (which includes the liberty to *not* associate with someone, even for reasons others would consider “bad”) and the liberty of exit (like, um, secession) too seriously for him to ever be anything other than persona non grata among those who want to ingratiate themselves in fashionable circles and say “we’re not so bad, we like the same things you do.”
Such people will only ever advocate “freedom of association” in socially acceptable ways (i.e. Caplan’s crusade for open borders, a cause that is in with the in crowd) and will avoid like the plague being active on things that would bring down the ire of the establishment. Whether this is “useful” or not depends upon where you sit, of course: it’s certainly socially useful to be “I’m not that kind of libertarian” (or is it? I notice Salon still lumps the Reason guys – who are decent fellows – in with “the h8ers” – regardless of how much effort they make). But does it ultimately help the cause of libertarianism (advancing liberty) to fill up the society with people who…well, lets just say that Progressives are correct about where their political interests lay in changing the demographics of various nations. Which, if they are correct about where their political interests lay (and they are) means that it is self-defeating (to put it mildly) for libertarians to support their causes.
I’ll also add that there are…personal reasons…dating back to unfortunate inter-libertarian spats in the early ’80s for why the Cato & Reason (and George Mason) faction of Libertarianism made Rothbard a “unperson” in their set. You’d think that after Rothbard died they’d get over these personal grudges and inter-Libertarian spats, but, evidently not. (Tom Woods & Lou Rockwell touch on some of these from the LvMI side of it all here in a short chat.