An example I often use to demonstrate the emptiness of negative freedom is to imagine that you are stuck down a well. (Suppose that you are there through no fault of your own. One day you just woke up, and found yourself stuck down a well. Perhaps a freak wind deposited you there.) Now, the libertarian seems to suggest that you are perfectly free so long as everybody else leaves you alone, as that way you suffer no interference. But surely we can see that this is mistaken. If left alone, you will dwindle and die. That's not any sort of freedom worth having. Substantive freedom requires that you be rescued from the well -- until that happens, you do not have any real opportunities open to you. And that is clearly what really matters.
Of course, once a libertarian realises this, they will be forced towards liberalism. It is not enough to leave poor children alone: by letting them starve, we do not thereby make them "free" in any worthwhile sense. The fulfilment of basic needs is a prerequisite to any form of freedom worth having. And, on top of that, education and parental love are necessary to a child's development into a fully autonomous and flourishing human being. (And, again, this is surely what matters.) Granted, the state cannot provide parental love - though it might help enable it, through family support and provisions for parental leave, etc. But anyone who genuinely values freedom ought to support making high quality education freely accessible to all - and not just to children of the rich.
But I digress. My main point here is philosophical rather than political. Even if you argue about the details, you must agree that it is substantive freedom that really matters. Elizabeth Anderson offers another example to reinforce this idea:
If the only kind of freedom that matters is that no one intentionally interfere with one's formal freedom of action, and not that one's opportunity set be large and full of worthwhile options, then freedom-lovers would have to oppose traffic laws, stop lights, and so forth, for interfering with freedom of movement. The result of a lack of such laws, however, is not actual freedom of movement, but, in areas of high traffic density, gridlock. (And, in areas of high traffic flow, grave danger.) To be sure, in a state of gridlock, one has the formal freedom to choose any movement in one's opportunity set--which amounts to being able to rock forward and back a couple of inches from bumper to bumper, getting nowhere. Some freedom! By contrast, if we give up certain formal freedoms--to run red lights and stop signs, to drive indiscriminately across lanes--we get in return a vastly expanded opportunity set, including the ability to actually get to places one wants to go, more safely and quickly than if we hadn't given up those freedoms. The point of formal freedom of movement--the right to move around, without coercive inteference by the state or other people--is that it is instrumental to expanding actual opportunities to move around where one wants to go. Merely formal freedom of movement, with nowhere to move to, or nowhere worth moving to, is not an end in itself.
Moreover, libertarians aren't even consistent here. As I've previously argued, property laws are a restriction on formal freedom, so the consistent libertarian ought to oppose the institution of property. Anderson notes this as well:
[P]rivate property essentially involves securing the owner's opportunity freedom at the expense of everyone else's freedom-as-non-interference. This has to be a losing argument, if freedom-as-non-interference is the freedom that matters. For private property essentially involves the use of coercive power to exclude others from using it. It essentially involves coercive interference, or the threat of interference, with everyone else. Common property in the earth and in things does not have this feature. Viewed from the static point of view of freedom-as-non-interference, the institution of private property involves a net loss of freedom.
(She then adds, in case it wasn't obvious, "This is a reductio of the conception of freedom as non-interference as the fundamental measure of freedom, not an attack on private property.")
So, given all of this, could someone please explain to me why anybody would ever think that mere negative freedom is somehow the most fundamental value? Because to me that position looks an awful lot like sheer idiocy.