LIke others have mentioned, I'm slightly cheesed at the way you expressed your question. Essentially, you implied that believing in Christianity is the norm, and those of who aren't "with it" are the ones who need to explain. I would argue that it is the Christians who need to explain their belief, if anybody asked. This is the inherent paradox in yelling at people not to preach: the burden of proof is on them. Unfortunately, far too often most of them don't even know why they believe what they do. I've known a few extraordinarily intelligent Christians, but every time it always comes back to faith.
Faith - belief in something without evidence.
Even this is misused sometimes. I've often heard when a logical contradiction is pointed out, "Well, you just have to have faith!" Faith is NOT belief in direct contradiction to evidence, faith is belief in the absence of evidence. Succinctly, absence of evidence is not evidence of absence. The Christian religion holds many logical contradictions as dogma, and refuses to try and explain them. Everyone's always got the "God blessed us" or "my guardian angel was watching me" for strange occurrences, but when a contradiction is posed "God works in mysterious ways" and "Who can know the mind of God?" Non-believers (and even believers sometimes) are chastised for anthropomorphizing God's intentions/actions, but those whose fingers wag almost always seem to have similar problems, at least on different topics.
I'm torn as to the societal aspect of religion, at least today. Certainly, one cannot use the South Park Mormon defense as an argument defending religion on base principals- just because something makes you feel better or helps get you through the day is not something to base a system of beliefs on. However, as a cultural institution, there is no doubt that religion has had both positive and negative aspects. Seemingly without trying one can recall the Crusades, the Spanish Inquisition, and even the sex scandals within the Cathollic Church: certainly things we don't want to be condoning and actively trying to stop whenever they arise. But just as easily we can think of all those who have lost loved ones, or gotten through particularly difficult ordeals using an incarnation of the "Footprints" poem. I do think that religious belief can be harmful to the human pysche, in that acts as a sort of cocoon around the person, especially in the aforementioned case of losing those close to us. It can protect the person from the feeling that the dead are actually gone, in a way that is much more dangerous than the "they'll always live in our hearts" line that is always given. The memory of a person can stay with us as long as we can remember them. The idea that they are floating on clouds in a mystical heaven, waiting to be reunited is nonsense. But is it harmless nonsense? What is the advantage of truly admitting that there is no life after this one, this is all you've got. Certainly, you can live your life the way you want without fear of divine or eternal reprecussions, but if that's how you choose how to live your life, who am I to judge? I would be imposing the same things that I despise about this "Christian nation" of ours upon another, and I already know how angry I get when it happens to me.
Woah, I got way off topic. Oh well, right forum right?