The Calvinist believes that prayer is useful. We pray because God commands us to. God DOES answer our prayer. He ordains that we should pray and he ordains that he will answer our prayers. By so doing God is able to both be good to us by answering our prayers and glorify his goodness in showing himself to be a prayer answering God. This includes praying for the conversion of sinners. Now, in your system as an Arminian or "non-reformed", why pray for the salvation of sinners? God WILL not override their free will. AND God WILL not draw one more than the other. AND God is ALREADY doing ALL that he can to save EVERYONE. What good is your prayer?