The best example of a public good is national defense. Herein lies the rub. Unfortunately, every individual realizes that they are better off not paying for defense and so no one does, leaving the society undefended. And it goes without saying that one of the many problems facing an undefended society is that it will have a hard time resisting attacks by other groups seeking to restrict its freedoms. Clearly then, because no freedom can be maintained with a pure libertarian defense plan, a mandatory tax should be instituted for the sake of freedom.
Similar arguments are applicable to many other goods.
Take public education, for instance. The government has decreed that, regardless of whether you have children who use the public schools, you still are required by law to pay taxes for education. The last point I want to make about libertarianism is that, within it, all freedoms are not equal. That is, the same freedoms granted to people with different backgrounds and skills may lead to vastly different levels of empowerment. And surely, given that freedom is the only currency which truly matters in a libertarian society, libertarians should be concerned that every citizen has at least an equal opportunity to express her freedoms.