The "free will defense" is a response to the problem of evil. Namely, a benevolent omnipotent god allows evil to exist in the world because it is necessary in a world with free will. God apparently believes free will is worth the evil. The corollary is that we live in the best of all possible worlds that God can create without defying human free will.
Another corollary is that evil should be roughly constant in time. People had just as much free will several centuries ago as we do now. We live in the best of all possible worlds with free will, and so did they. Therefore, we'd expect that our world is about as good as the world several centuries ago. I don't think that's what the actual world looks like.
In the context of Christian beliefs, I'm also confused how it's supposed to fit in with heaven. Ideas about heaven are quite diverse, but I gather that it's supposed to be a world that is better than our world.
One possibility is that heaven has free will. But that implies that our world is not the best of all possible worlds with free will. The alternative is that heaven does not have free will. But if free will is so important on Earth, why isn't it important in heaven?
(These are intended to be a "light" or "fun" arguments against the free will defense, not an especially serious ones. For reference, I've previously discussed the problem of evil.)