What are your thoughts on arguments against the Luminous Mysteries of the rosary?

There is no reason to argue against them. The rosary was made by man, not sent down from heaven; St. Benedict did a good job starting it, but it is a man-made devotion, holy because of the prayers attached to it, not because it was divinely ordained. So if a pope wants to add mysteries to enhance the prayers, he can, and the Luminous Mysteries are good mysteries: he did not draw them from some council, he took them right from the Gospel. So how can anyone argue against them just because a modern pope added five more mysteries? We have better things to worry about. If they really wanted to improve the rosary, I would add a devotion to the Immaculate Conception, and to the Assumption, because it is a rosary about Mary and her greatest gift was her Immaculate Conception, so that she could bear a sinless Jesus.