Michael Horton's The Christian Faith: A New Systematic Theology for Pilgrims on the Way, (Zondervan, 2011) is my 'big read' at the moment, see here. Studying his treatment of God's incommunicable attributes (Chapter 6) led me to jot down some thoughts:
Since God transcends space and time, he is free to be present with us in all places and at all times.
In confessing the impassibility of God we do not deny him emotions, but emotional spasms. He loves without sentiment and burns with wrath against sin without the least irritation. He is free to reach out to us in our suffering without being overcome by it.
Divine impassibility is the grounds of God's covenant faithfulness. His self-generated and eternal love cannot be stretched to breaking point by the failings of his chosen people. In his impassibility God is never discouraged or disappointed. Nothing can quench his determination to save hopeless sinners.
Only an impassible Father could have spared not his own Son, but delivered him up for us all at Calvary. That does not mean the Father was indifferent to the suffering of his Son, but that he was not so overwhelmed by it that he had to spare him the Cross without which we could not be spared. Impassible love is not needy and vulnerable, but free and outgoing; flowing from the Father, through the Son and by the Spirit to the world. No impassible God, no passion of Christ.