A good mission statement helps a church articulate its purpose and define its focus. A mission-driven church will focus on what it does best. It will accentrate its uniqueness. You should concentrate on what you do best and keep on doing it well. When I served in the Armed Forces, during the Korean conflict, I saw this verse in action.
Soldiers think first of their mission to please their commanding officer.
Everything else is secondary. I believe the same goes for a church. The mission of a church should drive everything else in the organization. Churches should be mission driven. A strong mission-driven church finds direction in its mission statement.