"Plumbline Libertarianism" Pro and Con
Those familiar with Walter Block should know that he advocates taking what he calls a "plumbline" approach to libertarianism that is neutral to the left/right scale or dichtomy. At face value, I agree with this if one is refering to the warped way in which the left/right scale is commonly construed in mainstream politics, since such a political spectrum cannot practically take libertarianism or anarchism into account. And while the Nolan Chart is certainly an improvement, I also have some problems with it due to the use of a dichotomy between economic and personal liberty.
The essence of plumbline libertarianism is that it subsumes anything that is voluntary and that the "tent" of the libertarian movement can theoretically be open enough to accomodate a wide range of groups, and at face value I agree with this as it is basically the equivolent of anarchism without adjectives. There is a degree of overlap between libertarianism and various other positions and there is a wide array of personal preferances that can be put into a libertarian context. On the other hand, the "tent" of the libertarian movement is supposed to be narrow or closed insofar as it's a question of voluntary interaction vs. coercion, and this is supposed to be represented by "the plumbline".
All this being granted, there are some serious problems that arise for someone trying to take such an approach to libertarianism, because one has to properly identify precisely where the plumbline is and what constitutes coercion, and it is at such a point that the internal divides of the libertarian movement become increasingly relevant. Various libertarians have completely different conceptions of where the plumbline starts and ends, and consequentially they have completely different conceptions of who belongs or doesn't belong in the libertarian movement. Someone could claim to be a plumbline libertarian and yet be rather partisan or incorrectly biased in terms of where they draw the lines.
There also may be a danger of plumbline libertarianism devolving into an oversimplication or a "thin" libertarianism that treats the status quo as being more voluntary or just than it actually is and brushes aside all concerns that don't directly relate to the use of force (although it may indirectly relate to it). Some of the vulgar libertarians seem convinced that they are simply remaining true to "the plumbline", but they are actually misusing libertarian theory as apologetics for currently existing structures and relationships in the economy. It is not true that libertarianism has nothing to say about anything other than the state.
While I don't believe that you need to have the exact same epistemology or metaphysics as me to be a decent libertarian, I think it's important to emphasize that the philosophical presuppositions that are used to lead to libertarianism are not irrelevant because the "libertarianism" that they lead to may not necessarily be the exact same "libertarianism". What positions one held prior to becoming a libertarian are also relevant, as they may still be reflected in someone's interpretation or understanding of libertarianism. People tend to still cling to biases held prior to their introduction to libertarianism.
In summary, I think that the notion of a plumbline libertarianism is sensible and noble at face value but it also poses certain dangers and it must be properly grounded in order to make sense. If it is not properly grounded, then it functions disingenously as a mask or cover for something more partisan or biased than what is being claimed.