People argue about religion for the same reason they argue about any other subject: ideas are important to man. Man depends on his concepts in his interactions with the world, and the degree of his success in life is the degree with which his concepts reflect reality. People know this implicitly, and their automatic emotional response to learning of their own errors is negative because of what this implies about their fitness to live.
To avoid finding himself in error, man can take one of two approaches. Some men correct their errors mercilessly and make pursuit of knowledge a primary goal in their lives. Other men dismiss the importance of knowledge and reason and prioritize evasions. Men need a set of beliefs from which to act, but since the latter gave up their rational faculty, they now have no choice but to absorb their beliefs by chance: from people around them, from mass media, from wishful thinking. The would-be tyrants of man's mind provide them with ready-to-wear excuses. Knowledge is picked up "intuitively," they purr. "Knowledge is relative," insist others. "Knowledge is impossible," proclaim still others.
Argument about religion is a symptom of a much greater issue: it is a sign of a man who has abandoned his mind and is looking for moral sanction for his action. He feels terror at his inability to deal with reality, he feels (rightly) that his mind cannot provide him with what he needs to survive, but he's too terrified to correct his prior mistakes: he sees his mind as impotent, reality as unknowable. He proclaims that "mystical" knowledge, or "intuitive," or "a priori" knowledge is what counts. Or he proclaims that there is only "relative" or "subjective" knowledge--or no knowledge at all--parroting the tyrants who offer those bromides to him under the auspices of the science which is supposed to enable man to reason: philosophy.
There are of course good reasons to argue. If a man discovers something which he judges as good, his benevolence might encourage him to share his find with other men. A man might realize that there are others who are willing to trade their knowledge for his by means of conversation and argument, and he will seek out those other men of honesty and integrity. When a man of honesty meets a man whose purpose in life is to evade the knowledge that he's unfit to live, his disdain for the latter is appropriate. But it is important to identify the essentials in this matter: what a man of honesty and integrity opposes are men who have abandoned their mind--whose only means of survival is by manipulating those who haven't--he does not oppose religious argument on the grounds that a participant is refusing to grant equal validity to conflicting claims. After all, granting equal validity to truth and falsehood is just another form of denying reality and of abdicating one's mind.