Perhaps first and foremost, our culture in America no longer has the "home field advantage" of everyone holding to a Christian worldview. For church leaders who want to take their congregations on the journey of reaching and changing people, formal training in seminary can prepare their minds through theological courses and their hearts through the study of church history.
I realize some people are suspicious of seminary because they feel it institutionalizes the work of ministry. I would say to them, "You're right. Sometimes it does do that." Seminary can make us into ministry professionals who are more concerned with the organization of congregational life than the people of the church. But I would push back against that attitude and say that most graduates will tell you the education they received has helped to shape a proper theology, ecclesiology, and missiology.
Seminary also offers something personal that we tend to lose as pastors and planters: camaraderie. As a planter, I was constantly around people and, at the same time, I was utterly alone. The time spent in seminary gives you the opportunity to learn how to seek out strong friendships and remain accountable in them. It is a side benefit to the education but a helpful one nonetheless.
That being said, I think that seminaries alone cannot provide all the pastors and church planters needed to reach North America and the world for the gospel.
We need to give people permission to pastor and plant churches even if they have not been to seminary. And, I believe, seminaries need to find ways to help equip those already on the field serving in mission and ministry.