That's how movements work; you start with an idea or principle and base membership of the movement on that, but rigid fidelity to that idea may prove socially unacceptable and may hinder your attempts to grow the movement (which is at least part of the point of the thing). So you blur the core idea a little.
The Catholic Church fudges on all kinds of things - belief in ancestral spirits, for instance, which is directly counter to Catholic dogma - in order to be accepted in many parts of the world. Zoroastrianism - a religion founded by Zarathushtra on the idea of opposition to the unnecessary slaughter of cattle - adopted the common Near Eastern trope of sacrificing cattle because it was easier to do this and gain adherents than uphold the original principle and reduce the potential number of followers. Movements have to balance the principles of the movement with the desire to increase the number of followers of it. This simple fact makes a social movement intellectually unsatisfying, however good it otherwise feels to be a member.
I'm about as suspicious of the 'sceptic' movement (or 'skeptic' movement, if you're American) as I am of anything else, despite agreeing with the general principles of the thing. I've subscribed to Pharyngula for years, I used to read James Randi's newsletters (Swift, unfortunately ended long ago), and I get email updates from the Bad Astronomer, Michael Shermer, Robert Park, and so many of the other things associated with this movement. I've even subscribed to Freethought Blogs (even though it is now completely different to the science and sceptic blogs I used to read on ScienceBlogs). I even watch, and sometimes fumingly disagree with, Penn & Teller's Bullshit. I've never given any money to any sceptic fundraiser, but it's a movement I agree with, in general: increasing reasonable thought about the world. Likewise the atheist 'movement'. I agree with the central tenets - there isn't, after all, a god - but I'm not too fussed about getting involved, and I'm suspicious of it as a movement.
These movements have changed a lot since I've been observing them, in the space of a decade or so. This may be due to the internet, and the massive increase in followers it provides for almost any movement. But the problem has always been there, I suspect. The problem - I see it as a problem, while you might not - is that the whole thing is about the movement itself, not the idea behind it.
Here's what I mean: instead of reading a scientific book on an interesting subject, like supercontinents or evolutionary biology or a human history of the earth, many sceptics - I know because I've seen their reading lists - would prefer to read books focused on debunking common misconceptions. Instead of reading about world history out of curiosity, they would prefer to read a book that attempts to debunk the 'ancient alien' theory of world history. Instead of reading about cognitive science, they would rather read about the cognitive biases behind mistaken beliefs, like belief in the Loch Ness monster or Christianity or whatever else. They would rather watch 'sceptical' television documentaries, like Bullshit, instead of scientific documentaries (even though, thinking about it, they should really be the same thing).
They'd rather read about why Ancient Aliens is wrong than about the subjects Ancient Aliens distorts. The fundamental point - increasing rational thought about all topics and promoting scientific inquiry - is enslaved by the necessity to tie it to some issue of prominence in the sceptic movement. I'm much more interested in the Tiwanaku civilisation as a fascinating ancient civilisation complete with bronze-working and refined palaces than as yet another thing Ancient Aliens got wrong. (I've often considered writing a history of the world that focuses on things Ancient Aliens got wrong to drum up readership, but frankly I'd feel the whole thing much too cheap to contemplate, and in any case I'm sure someone else could do it better.) This focus on correcting common misconceptions has a number of unfortunate consequences, not least of which is the fact that many of the ideas sceptics use to challenge the Bible, von Daeniken, or Ancient Aliens are themselves incorrect. I've seen countless articles and videos 'explaining' the Garden of Eden myth, or Atlantis, in terms of a hypothetical early Holocene flooding of the Persian Gulf, something which in fact never happened (the Gulf has probably had the same coastline for the past few dozen millennia at least).
One of the many logical fallacies sceptics and atheists like to trot out is the No True Scotsman fallacy. This is an interesting fallacy identified and named by Antony Flew, an acerbic twentieth century philosopher and atheist (who has since found a deist god). (Read the wiki page for details of the fallacy.) But the sceptic movement, like any movement based on an abstract concept, is designed to produce a No True Scotman result. The atheist movement likewise. There are endless debates about what the 'true' definition of atheism is, and it is pretty clear that different people 'come to atheism' due to different arguments and entirely different, often incommensurate, principles. I don't believe in a deity because the universe isn't the way it would be if a deity existed, and because religious arguments are almost always wrong and predicated on incorrect assumptions (especially fundamental ones, like the first cause). The clearly human derivation of morality also stands out as a good reason to believe that the universe is impersonal and ungodly. Other people 'became atheists' due to some calamity in their lives that forced them to reconsider the issue of god's existence in light of the problem of suffering. These are clearly very different ideas and respond to entirely different issues, and are united only by common use of the word 'atheist'.
This is quite necessary for a social movement; such movements have to focus around flexible concepts or they won't acquire followers, so the atheist movement actually involves people who believe completely different things about the central topic. Do atheists deny the existence of god? Apparently that presupposes the existence of a god that atheists deny, so that's out. Or is it? Some atheists in the atheist movement do deny the existence of god. Do atheists believe that there is no god? So most people think - but how strongly? Do they deny gods outright? Do they leave a little room for doubt, because they want to be sceptical and rational rather than dogmatic?
There isn't any single answer to those questions, because atheism is just a word. What matters is what the individuals believe, not what the word is held to mean. It is clearly possible to believe a number of different things and label oneself an 'atheist' or a 'sceptic', and so the word gives only vague indication as to the purpose of the movement or the beliefs of the individual. And instead of saying,
'I believe that p, which some people call atheism'
they say (in essence),
'I'm an atheist, therefore I believe that p'.
This is, as I say, almost designed to produce a No True Scotsman fallacy, especially when combined with the out-group hostility of your typical social movement (the sceptic and atheist movements are no exception). Instead of deciding on the beliefs before choosing the label, the label becomes the centre around which the beliefs flow (and the label, used (of course) by the movement, becomes synonymous with it in some degree). This is advantageous for the people organising the movement as they can use this flexibility in definition to produce a big-tent movement, increasing revenue at events and the prestige of those who run them. But it produces a contradiction when dealing with scepticism and atheism, because those movements are supposedly based on principles that directly run against No True Scotsman thinking.
Research has been conducted on Star Trek fans a number of times. Trekkies are pretty interesting people because Star Trek is actually quite an interesting programme. It portrays a world free of many of the ills that blight our current one, a global, indeed universal, humanitarian world in which nation states have disappeared and ethnocentric racism is a thing of the past. Gene Roddenberry even commented that Picard's baldness wasn't a problem because in the time of Star Trek: TNG, they simply wouldn't care. That sounds like an ideal world (on the other hand, the unification of global culture and the obsolescence of languages other than English seems a little problematic!). I am not attached to nations and I find the arbitrary division of the planet into ethnic groups and populations dismaying and horrifying. Apparently there are other people who feel the same way, and many of these people watch Star Trek.
Trekkies, according to social scientists who have studied them, like to claim that their meetings are about moving the world forward - about producing the very world of the TV show. They tell researchers that the movement is based on humanitarian aims, using Star Trek as a model for how to move ahead and what to aim for. That sounds quite noble to me - except that what actually happens at Star Trek conventions and meetings in the pub is that members discuss their favourite episodes, argue about which captain is best, and generally avoid substantive social change, despite that being the claimed aim of many of them.
The sceptic and atheist movements seem a bit like that. Instead of discussing science or real philosophy of religion in much detail, many sceptics and atheists learn these topics in order to undermine the arguments of their interlocutors or to feel like they're part of the movement. You can see this even in the blogosphere, where scientific issues are rarely discussed in depth but squabbles in the movement receive a huge amount of coverage (and when scientific issues are discussed, they usually receive fewer hits than the inflammatory posts). Atheists don't discuss the philosophy of religion - the beliefs they actually have about the existence of deities - very often, and it would be quite difficult to base a movement on this. Instead, they discuss why other people are wrong.
The ideas behind the sceptic and atheist movements are noble and they should be pursued. But the movements themselves are like social movements of all kinds: riven with divisions, engaged with not for the ideas themselves but for what the ideas can do, and distorted into amalgams of beliefs held together only by use of a common label.
I'm not a 'faitheist' (someone who believes no god exists, but who thinks religion/faith is good in general) or an 'accommodationist' (someone who believes science and religion are commensurate). I don't believe religions deserve any privileges, and I don't believe in astrology, spiritualism, ancient aliens, reflexology, chiropractic, crystal healing, new age medicines, or faith healing. I'm not arguing for a change of tone and I don't think sceptics should be less vociferous or outspoken. I just think that following, or leading, a social movement based on an abstract concept is a road to contradiction and silliness, not a road to good sense.