One of the more moronic arguments made against atheists is to ask why we care so much about something we don't believe in. Dinesh D'Souza has a particular fondness for this argument, usually couching it in terms like "I don't believe in unicorns, but I'm not writing books about why I don't believe in them." Yes DD, and there also happen not to be people in your culture beating you over the head with unicornisms, trying to convert you to unicornism, and limiting your freedoms because of what they think the big Unicorn in the Sky wants.
For an impassioned, detailed explanation of why it all matters, check out this video from a little energetic chatterbox who youtube's by "gogreen". Oh, and if her cleavage bothers you, watch this one