Say that moral freedom is the ability to choose between a morally permissible and a morally wrong action. Perfect freedom, on the other hand, is a freedom to choose between morally permissible actions, but with a perfect and infallible directedness at the good of the sort that God and the saints in heaven are said to enjoy.
Morriston (and others before him, like Quentin Smith, but Morriston's piece is particularly well developed) basically offers this dilemma: Either moral freedom is better than than perfect freedom or not. If moral freedom is better, then God has the less valuable kind of freedom, which seems incompatible with God's perfection. If moral freedom is not better, then God should have created beings with perfect freedom, since this way all the evils flowing from our misuse of moral freedom would have been prevented.
I want to make two points. First, the relevant question shouldn't be whether moral freedom is better than perfect freedom, but whether the action of creating beings with moral freedom is better than the action of creating beings with perfect freedom. An action can be better than another, even if its intended effect is no better. For instance, if I promised an editor a paper on modality, and I have the time for only one paper, the action of writing a paper on modality is better than the action of writing a paper on the Trinity, even if the effect of the latter action may be the better.
With this distinction in mind, one notices that there is a difference in value between God's creating a being that inevitably loves him back and his creating a being that gets to choose whether or not to love him back. Even if a being that inevitably loves him back is no better, God's action of inviting someone into communion with him very much has something very significant to be said for it that God's creating someone who will inevitably be in communion with him doesn't.
The second point is this. There is a value to loving someone by choice. Now when God and St Francis love each other, each loves the other by choice. Francis chooses to love God, while being able not to. But God likewise chooses to love Francis, while being able not to. Now you might say: "But doesn't God have to love everyone, given that he is love itself?" I agree (though I know some don't): necessarily, if Francis exists, God loves him. But Francis doesn't have to exist—Francis only exists because God chose to create him. Thus God has freedom whether to love Francis, a freedom he exhibits in choosing to create Francis, something he did not have to do.
Now there is a necessary asymmetry here. Since we cannot have a choice about whether God exists, and once God exists, there is the obligation to love him, our choice requires moral freedom: it is a choice between the good of love and the evil of not loving the supremely lovable God. But for God the choice whether to love Francis was at the same time a choice whether to create Francis. This choice does not require moral freedom, since it is not a choice between good and evil, but only good and good-or-neutral.
So on both sides, the relationship between God and Francis involves a freedom to love or not to love Francis. This freedom is valuable and God has it. But in Francis this freedom, of necessity, is moral freedom. So it is not that moral freedom is more valuable than perfect freedom. Rather, it is that in a creature, freedom whether to love God has to be an instance of moral freedom, while in God, freedom whether to love a creature is an instance of perfect freedom.
Objection: But doesn't Morriston's problem come back when we consider the doctrine of the Trinity? The Father and the Son do not choose to love each over not loving each other. The Son is not a creature, and so the Father does not choose to create the Son rather than the Father. Yet, surely, the intra-Trinitarian love is the most perfect kind of love. So wouldn't a creature that has to love God have a better kind of love than one that has a choice about it?
Response 1: A certain symmetry and equality in love are particularly valuable. In the Trinity, we have a symmetry: no Person of the Trinity has the freedom to fail to love another. But we automatically start off with God having a choice whether to be in a relationship of love with a creature, namely through his having a choice whether to create the creature. It makes for deeper equality and symmetry if the creature also has a choice about how to respond to God.
A love relationship that is chosen on one side but not on another is less valuable through the asymmetry. Imagine a woman who chose to have a baby had a drug that would ensure that the child would love her back. She had a choice, to some degree, whether to love the baby. But she refuses the child a choice about whether and how to reciprocate the relationship.
Response 2: To choose to love makes one intimately related to one's love. But in the Trinitarian case, there is an even deeper relation to love: God is identical with his love.