The issue of prayer is not God needing His people to plead with him enough to convince him to act. Rather, the issue is dependence. The quality of our prayer lives is directly proportional to how utterly unable we (and those engaged in missions) are bring about substantial advance for God's Kingdom. The more we depend on our skills and gifts, the less we depend on God. As Ray Ortlund has said, "Doing what we can on our own with our own brilliance and savvy is the exact opposite of what God can do."
Missions is arguably the ultimate battle against powers and principalities rather than flesh and blood (Eph. 6.12). Prayer is the most critical weapon we bring to this battle.