Hmm. Well... I'm not exactly sure how to address your questions, mainly because it seems like you're approaching atheism from a theist mindset, which doesn't quite match up with how atheists look at things. But, at the risk of sounding like I'm not actually answering the question, I'll put a few things out there anyway, in the hopes that it might clear things up a bit.
Atheism is not a religion. Yes, I realize that CF classifies it as such for convenience, but it isn't actually a religion. Atheism is nothing more or less than a position on the existence or non-existence of a god or gods. Theists believe in a god or gods. Atheists do not. Period.
Beyond that, given that one's thoughts about god will end up coloring one's approach to the universe a lot of the time, you will often find similarities of belief among atheists. A lack of belief in a deity means that an atheist will not include a deity in their beliefs about the origins of the universe, for example.
Nonetheless, despite some commonly held ideas, there is no inbuilt atheist hierarchy. There is no unifying set of rules and there are no required beliefs, beyond "atheists do not have a belief in a god or gods." Consequently, even the commonly held values are not absolutes by a long shot. There is no atheist missionary program either, though one guy from Australia did go door-to-door as a kind of one-shot psychological experiment. (He videotaped him and his buddy and the responses they got. One old guy hit them with a rake, he was so angry at their disbelief.)
That includes the value of whether or not to be outspoken about one's atheism. And if so, where and when it's appropriate to be that way, and why.
Atheists (myself included) visit sites like this for a variety of reasons. Some of us are seeking to understand the religion which dominates our culture by interacting with members of that religion directly. Some of us are finding ourselves among the
most vilified members of our society, and we go public in an effort to combat misperceptions about who and what we are. Some of us are just argumentative by nature. Still others take issue with the increasing efforts many theists make to erode people's rights (or interfere with, say, science education), and speak up as a way of fighting back.
Others believe in challenging beliefs which we find spurious and unjustified. Still others don't give two shakes of a rat's tail what anybody thinks of them, and spend their lives quietly living under everyone's radar. You'd never know such people were atheists, if you didn't ask.
The one thing that I find a bit unsettling about questions like "Why are you speaking up; you're just wasting your time" is that there is an implication that atheists should shut up and mind our own business, and not "bother" anyone else with our opinions.
That would certainly help theists preserve their own faith, because then no one would ever challenge it. But I say, if a believer is really interested in finding out the truth, then they are not served by living in a theological vacuum. It's easy to believe in something if that's all everyone around you tells you is true. I should probably also note that I was raised to value the free and open exchange of ideas, so I am not a fan of censorship in any form (even though I put up with it under certain circumstances). If someone is uncomfortable with my being an atheist and talking about it, I think that speaks more about their own fears than it does with any right I might have to talk openly about what I believe.
I hope that helps a bit.
Good point Tyndale 
