"How things have been done" does not equal "how they should be done". You make the (false) assumption that if these things were not paid for by taxes, then they would not be paid for at all. And the Tragedy of the Commons argument doesn't hold water. It is a challenge to be surmounted, but not a damning mark on the possibility of voluntarily-provided services. Behold:
Tragedies of the Commons have the potential to exist wherever there is a universally accessible good or service, i.e. roads. However, governmental power itself must be considered such a "public good", in that it was theoretically available to all US citizens, but has clearly been "overgrazed" by just a handful of "shepherds", to return the metaphor to its orignal context. So, government cannot be the solution to ToC since it is the good which is MOST SUBJECT to ToC!
Here is just a little sample of how something like defense could be provided in a non-coercive way. In the future, there are various defense agencies in the geographical region we now call the US. You will not be compelled to contribute to this service, however those who do get their names put in secure databases. Now suppose you apply for a job somewhere; since people value this defense service, they will be preferential towards those who are not "free riders". They can ask for your permission to query the database to see whether you support defense. If you have not, they may choose not to hire you. Now there is merely a social and peaceful pressure being put on individuals to support these things.
It is very telling when people worry that a service wouldn't be provided without the state, but their concern is the very proof that it would be done! In fact, those services are almost certainly being done quite poorly BECAUSE of the state. The state has no motivation to follow through.+Jarrod Bolin +John Boston +Robert Pearce
If you think that these things can only be provided by such coercive means, read the free book Practical Anarchy here: http://www.freedomainradio.com/FreeBooks.aspx
Once again, "it is" does not mean "it should be".