In some recent dialogue with Darrell Guder, editor of the Missional Church, the book that brought "missional" to the front of the conversation, he shared some thoughts on the phrase "missional church."
One of my students has described [the missional church] as a "scaffolding term." It should not be necessary to speak of the "missional church," but since it is possible for a church to exist that does not understand itself as defined by God's mission, the term is needed.
We shouldn't have to say "missional ecclesiology" or "missional theology," but the plain fact is that the church in the West has been doing ecclesiology (and theology in general!) for centuries and ignoring its fundamental and comprehensive mission while doing so. Perhaps there will be day when we talk about theology, or church, or witness, or scriptural interpretation, and it will be unquestioned that what we are talking about is defined by God's mission and our vocation. Then the term, like a scaffolding after completing the job, can be dropped. But we are a long time from that point, I fear.
I think that the term should be unnecessary (like the phrase "biblical preaching," might I add). Yet, we add the modifier because it is needed to refocus the church on the mission of God.