It is better to be than to not-be
At Mass this Sunday, Father Al began his homily with a few comments on Jesus' words in John 6:44: "No one can come to me unless the Father who sent me draws him; and I will raise him up at the last day" (the Gospel reading was John 6:41-51). Father noted (and I'm paraphrasing here) that at this point in his life, he understands most "mysterious" things, with a few things excepted. One of those exceptions was this: if God knows from all eternity those souls who will finally refuse his gift of grace and salvation and choose eternal separation from Him instead, why did He create them?
Here is my meager attempt at a possible response.
Taking Genesis 1 as the point of departure ("and God saw that it was good"), it is a basic principle of Catholic theology and philosophy that God created all things good, and this fundamental goodness remains, despite the effects of the Fall of humanity in the garden. (Philosophically, this notion is expressed in the axiom that being and good are convertible.)
In other words, insofar as something exists (i.e. has being), it is good.
Consider, then, those who are damned (who count among their number the fallen angels and any human beings who persist in refusing God at the moment of their death). On a moral level, they are rightly described as evil. But on a metaphysical level, the fact that they continue to exist in some way, however diminished, means that they are good, and hence that their being adds some goodness to the cosmos.
And here lays my possible answer to Father's query: by the sheer fact of their being, the existence of even the damned "adds" some measure of goodness to creation. In other words, the universe is better off with them than it would have been without them.