Given that it has zero chance of attaining power or introducing its favoured policies - what is the harm of principled libertarianism?
Simply that it is destructive. It helps to destroy the traditional order, it helps to destroy hierarchy, it helps to destroy existing sources of countervailing power.
So, libertarians supply arguments which are used or exploited by Leftists with which to atomise society - to break down professions, guilds, unions, protections, privileges insofar as they help maintain the traditional (existing) order.
Libertarians want freedom of the individual from all forms of authority, but the only authorities they are actually allowed to subvert and usurp are those forms of authority which the Left dislikes. A one-sided liberation, always tending to assist socialism.
Libertarians help create the chaos and weakness which is then used by Leftists to create their statist, communist, socialist, politically correct society.
This happens because it is much, much, much easier to destroy Good than to create Good.
The libertarians (sincerely) believe that they are only destroying the Good in order to introduce something Better (better, that is, from the perspective of efficiency) but that doesn't actually happen, because libertarians are so weak.
They can accomplish the easy task of destruction, but they never accomplish the vastly more difficult task of making something better.
Note: I was a libertarian - mostly from the early 1990s to the mid 2000s; and although I considered myself standing in opposition to the Leftist and politically correct intellectual mainstream, and although I did meet up with some trouble and strife; I was remarkably tolerated, and it seemed remarkable to me at the time.
But, in fact, the tolerance was more than remarkable - it was sinister; in the literal sense that my libertarianism was being 'used' as a 'tool' of the Left that was directed and selectively deployed. When not useful to the Left, libertarianism was simply ignored or over-ruled.
However, against this background of Leftist tolerance for libertarianism (a tolerance based on self-confidence that libertarianism is helpless against the Left when it comes to power) there are periodically - at least annually - blood sacrifices of libertarians who make the mistake of applying their analysis to the politically correct taboos.
But libertarians who steer clear of the Leftist taboos are given almost limitless tolerance - serving the Left as ideological resources and providing a self-gratifying illusion of the Left tolerating dissent.