A common line of reasoning in response to the problem of evil is (very roughly) that free will is an exceedingly great good, but God can't give us this good without thereby preventing the possibility of our misusing it and causing evil in the world. My concern here is not whether the free will defense is a successful defeater for the problem of evil, but rather whether it fits well with a popular theistic view of meta-ethics, viz., divine command theory.
To see the worry, start with a crude version of divine command theory, according to which all moral properties, including both moral values and moral duties, are grounded in God's decrees. On such a view, God can confer moral value or moral worth on anything he likes by mere decree, in which case free will isn't intrinsically valuable, in which case God's ability to bring about the greatest goods isn't dependent upon creating creatures with free will, in which case the free will defense looks to be in big trouble.
One might reply that the problem raised above can be avoided by appealing to a more sophisticated version of divine command theory, such as the modified version developed and defended by Robert Adams and others. However, it's not at all clear that this will be of help to the theist in addressing our worry. For while it's true that moral value or moral worth doesn't depend upon God's will on this latter sort of view, it does depend upon God's nature, such that something has moral worth or moral goodness just to the extent that it resembles God's nature. But the problem is that God doesn't have the kind of free will in play in the free will defense. This is because God is essentially morally perfect, in which case there is no possible world at which God freely does something morally wrong. But if that's right, then the kind of free will attributed to humans in the free will defense doesn't resemble the kind of agency had by God. In fact, Adams-style modified divine command theory seems to have the implication that creatures with a kind of will incapable of performing morally wrong actions have greater moral value or worth than creatures that are capable of performing them. And if that's right, then the free will defense looks to be in just as much trouble when conjoined with modified divine command theory as it does when paired with the crude version.