What a middle school assessment of the opposition. Many of the atheists I know, myself included, are skeptics. We spend time arguing against other types of bunk like UFOs, Bigfoot, CAM, etc. By your middle school logic, it is apparent that we don't actually disbelieve these claims. If we really did not believe, then why would we waste our time arguing against things which we believe to be false? Because you fundamentally misunderstand. We do not argue against Bigfoot; we argue against the belief in Bigfoot. I don't believe Bigfoot exists anymore than I believe that the Christian God exists -- in fact, I positively believe those two things do not exist.
I, like skeptics and many atheists, argue against nonsense like Bigfoot and God because of a few reasons. One reason is that the belief is often harmful in some way to the person who holds it. Unlike the horrible painting you have created of atheists in your mind, we actually care about people -- to quote the great Dr. Comfort, we atheists actually "have a conscience." Belief in stuff like homeopathy is harmful because it acts as a horrible substitute for actual medicine -- on the same level that feces is a horrible substitute for chocolate: they may look similar, but one makes you feel good and the other one is crap.
Another reason, though, is that you misunderstand again. You recognize that this life is precious for us (another reason to be an atheist), so it is a logical conclusion that we would like to have meaningful, fulfilled lives and enjoy ourselves. Most atheists -- and I do mean most -- do not care at all about arguing against religion. They see it as a waste of their time as there's better things to do. One of my best friends is an atheist and we have fun laughing at silly (religious/supernatural) beliefs and creationist arguments, but that's as far as she goes. She doesn't want to discuss any philosophical or scientific arguments one way or the other.
That's great for her, because she does not enjoy it; she spends her time doing things she does enjoy. You fail to even consider, though, that arguing against religion and other nonsense is enjoyable to many atheists. That's the only reason an atheist would even have a blog dedicated to it.
Other reasons include the fact that we have religion forced down our throats in society. We are activists because you good Christians feel it appropriate to infringe on our rights. So, we take action in the government to protect ourselves. There's also many who have our relationships with our families poisoned by religion, and speaking out against it is a cathartic experience. There's also a passion for educating people. For example, creationists have launched a vile attack on science, and it's important for people to exist to counter it and explain why creationism fails at every level.
But these are secondary for me. I spend my time arguing against religion and religious superstition because it's enjoyable. I know it is a difficult concept to grasp for those who accept their religion on faith, but intellectualism is a prosperous exercise. Engaging in intellectual activities expands my own sphere. When I recognize how people delude themselves into many of the beliefs they have regarding religion, how people mistake anecdotes for rigorous evidence, how people use poor logic, it improves my mind.
I can use these observations and inspect my own beliefs on other matters critically. Am I forming these beliefs for good reasons? Am I analyzing the evidence, setting my bias aside? Am I carefully making sure I see all the variables in the evidence? Am I using proper logic? Can my same arguments be used for the contrary belief?