Prayer is pointless
let us assume the following things are true: a) God exists and b) God is perfect [thus implying that he always makes the best decision, no matter what].
EDIT Let us only think about non-worship prayer.
Third, let us set a scenario: There is a little girl sick with leukemia and her Christian parents and their friends from Church pray devoutly for her day in and day out for a week. They fast, keep vigil, read Bible verses, and ask God relentlessly to save the little girl's life. Three days later, the girl dies.
There is a question that must be raised: Why did the girl die, even after a week of prayer? The answer is a familiar one that Christians often give: it was not God's plan for her to live. We must remember that God is perfect, and because of this, he always makes the best decision--he never changes his plans written in that big Book of Life, no matter what, because he is perfect and he can never make a wrong decision. Therefore, there can never be a situation where God makes a mistake; he simply cannot change his infallible plans. So that is the first point: God has made his plans, and will can never change them.
Therefore, when a person prays, there are two possible scenarios: 1) the person prays for something that IS in God's plan, or 2) the person prays for something that IS NOT in God's plan. Shown above, one never knows which of the two one is praying for. We already know from the example of the little girl that praying for something that isn't in God's plan is worthless, because God will never change his plans. By logic, the first scenario--praying for something that IS in God's plan--is equally worthless: The prayer is already destined to come true, so what is the point in praying for it? If it's going to happen anyway, what's the point of praying for it.
You either pray and can't convince God to change his perfect plans or you pray and you're already getting what you're praying for. Either way, you won't be able to make any difference whatsoever, so the final argument is proven correct in my eyes: Prayer is pointless.