According to St. Thomas, we only have properties like goodness, wisdom and being by participation in divine goodness, divine wisdom and divine being, respectively. Divine goodness, divine wisdom and divine being are the focal cases of goodness, wisdom and being, respectively. We have these qualities only insofar as we are dependent on God's having their focal cases. Our goodness, our wisdom and our being are mere shadows, as Plato would say, which is why Jesus said that only God is good (Mark 10:18), and St. Catherine of Siena reports God as having told her: "I am he who is and you are she who is not."
I suspect that if we reflect on this, we will find an answer to the question of why it is that we add nothing to the value of God--the world, given that God exists, is in an overall sense no better for having us in it. God's moral goodness is no greater if he creates us than if he does not. This had better be true--what God has chosen to do for us is pure grace, and is in no way necessitated. (And if one thinks that to be truly good, one needs some kind of generation of good, the a-causal timeless begetting of the consubstantial Son by the Father, and the a-causal timeless procession of the consubstantial Holy Spirit from the Father through the Son, should help.)
All this should provide ample ammunition against Rowe's argument that God doesn't exist, because if he did, then for any world he created, he could have created a better, and hence he could be morally outdone. For God's moral worth is not dependent on what he creates; this worth is, simply, infinite, no matter what.