I do agree that we can say that buddhism is considered by many as a vast spectrum of ideas, beliefs, concepts etc. What I'm trying to defend is my opinion, that this approach is wrong. Wrong, because it makes difficult to discuss about the subject. If one is stretching the meaning of word 'buddhism' (or anything else) to the point where it can mean many different things, how can we really know what we are talking about? So can we assume that what we should call buddhism are all ideas, beliefs and concepts that have origin in Buddha's teachings but also that they should share some similarity with his words? I do agree that there is no clear line that can divide what belongs to buddhism and what doesn't, but at least that can give us some guidelines. According to them, we can be sure that zen is a form of buddhism as well as mentioned theravada, and yet my friend, who recently discovered buddhism and does not know much about Buddha, didn't change her way of living, her habits, views karma as something like experince points in rpg game and treats meditation like a yoga class - should not be called buddhist at all (and her view could be called very loose interpretation of buddhism).
Now, I don't know if I'm alone in this, but I see many similarities between Buddha and Jesus Christ. Both had followers and they initiated movements that quickly became religions. One was considered as a enlighted being, the other as a son of god - yet in their words they lacked integrity and had strange tendency to believe many superstitions and myths popular in theit times (shouldn't they be above them?). For Buddha a good example could be his relation to women - he didn't like the idea of women followers (he agreed eventually but gave them much more strict rules than for men) and he thought that they should focus on becoming a men in next life first (source - 'Buddha - Die Wege des Erleuchteten' by Helmut Uhlig, 1994). Boddha's as well as Jesus' philosophies and teachings laid the grounds for religions - that were organized and dogmatic. What I think is important to observe about any religion is that the actual teachings, 'words of wisdom' usually sound great and beatiful, but it has not much to do with reality of the organization. Let's look at Pali doctrine and concept of relying on own experinces. Sounds great! But according to the linked article, Theravada originated from Sthaviravada, other old school of buddhism in the times of Third Buddhist council (250BC) which was convened "to rid the Sangha of corruption and bogus monks who held heretical views" and "to ensure the Dhamma was kept pure". I can't see that much of philosophical freedom in this, can you?
And this goes on and on for centuries - they have holy books, teachers who know better, they build monasteries, go along with monarchs, they even go as far as to constitute buddhism as national religion and enjoy theocracy (vide buddhism in Tibet). Zen buddhists in Japan consider themselves as those who got Buddha's ideas very clearly, yet during 2nd world war their most esteemed teachers were able to convince their followers that (!) war and killing Chinese people is a good thing! ('Zen at War' - Brian Daizen Victoria, 1997) So yes, maybe in "theory" there is a room for personal experince and research, but in practice there are some concepts (like reincarnation, heaven and hell) that you can't debate or discuss. So yes, for more than 2000 years buddhism was a dogmatic religion. Maybe not to the degree like we see it in christianity, but it doesn't mean we can call it free from dogma in my opinion.
Let me repeat that again - today buddhism is considered by many as a vast spectrum of ideas, beliefs, concepts (especially by our west culture). But my point is that half of the Asia for 2k years did not share the same definition for 'buddhism'. So we can't just ignore that. Especially if we look into the reasons for such 'skewed' view in our eyes. I think it was Kappleu in 'Three Pillars of Zen' who wrote about why in 19th and 20th century academic researchers were interested in buddhism. Not because they were convinced that it could be really possible that we all are born again and again untill we can finally achieve nirvana or that they were looking for evidence where exactly heaven and hell are located. What was interesting for them were ascpects of buddhism that were similar to discoveries of our sience - in our psychology for example (ego, true self) or physics (quantum theory). For all these people who were (and are) disappointed by christianity, those were very interesting findings: "Yeah, looks like what our priests say about reality is not even close to the true, but hey, those monks in China and/or Japan, they say same things (at least it looks like) as our scientists. Maybe we don't have to throw away that spiritual irrational bullsh*t we believe in, we just have to pick a different one". I know I'm a little sarcastic here, but I really belive that was the spark that started the fire. So they stripped buddhism from things they didn't like and brought it to west culture - but as an intellectual play, not as a religion. Then people got hooked (can I say that? sorry me english bad), and discovered another 'magnet' - that buddhism offers a path, a way to spiritual enlightment in your own life, by your own experience. That's something really great in comparison to christianity ("wait till you die, and then you'll see we were right"). And I have to admit - it worked for me and pushed me toward buddhism. So the second wave of popularity was created by people who found in buddhism a way to spiritual development that is not restricted by some rules, so they can negotiate with dogma, follow their own experiences. And again - they had to strip anything that was against this ideal view. And then there was a third wave, when people were seeking "truth" from the source, so religious centers in Asia were accepting them as novices in their temples, so they could bring buddhism as religion to our culture. And they were sending their own teachers. Those three different views mixed with themselves and thats why - I believe - today there is so many different views on buddhism.
But I would like to focus on that personal development aspect, that SimonF considers similar to libertarian ideas. The way to discover the truth in buddism is a way through practice. Practice of meditiation, practice of zazen etc. Theravada says: "insight must come from the aspirant's experience, critical investigation, and reasoning instead of by blind faith". Key thing is that this experience, reason you can use in relation to your practice - not to the reality. In other words - you're free to rely on your experince regarding the practice, but it doesn't mean that by using the same principles outside the practice, you can still discover truth. So when by using logic, senses and reason you say that you don't remember your previous life and that proves there is no reincarnation, buddhists say: you're wrong cause you live in a illusion. But when you're meditating and suddenly have a real feeling, that you were a Russian soldier in WW2 fighting at Siege of Leningrad - then you're right and no one can argue that in fact you were a German.
But this way to serach for truth have some (obvious) flaws. Buddhist's practices are similar to those in other religions. Except some minor changes, buddhist monks are in many ways following the same path as catholic monks. They just have different names for similar procedures. And there was no single catholic monk, that during his stay in solitude or during long prayers suddenly discovered that true nature of reality is that we need to reborn again. How is that possible? Well, because there is no room for reason, logic when you're trying to discover truth about something not by researching it, but by looking for answer in some state of your own mind! So, in the absence of some kind of dogma, people tend to find ridicoulus answers (vide hundreds of strange sects that are based on practises like meditation, reading dreams, astral journeys etc) and that is why dogma is preferred by majority - so in case of some strange experinces during 8 hour of sitting in zazen, you will know how to interpret it correctly.
That is why I don't see parrarel principles and I don't belive Buddhism is very compatible with libertarian and voluntaryist ideals.
PS: Sorry, I guess it's way too long to read, but I tried to explain my views, reply as good as I can and it looks like I've lost control. Sorry for mistakes in english, im Polish and have to polish my english ;)