That isn't what he is saying. How can you accuse him of thinking the old testament is useless when he believes that it leads people to Jesus Christ? After all, its a school master, or tutor. In it Jesus Christ is revealed though hidden throughout. That is a false assumption you are making against him, one that he doesn't subscribe to. I am sure he considers it valuable, it just isn't valuable to make anyone righteous. It is holy, good and just but has no ability to make us so. He understands that this happens, the overcoming of sin, through grace and not Law as the scriptures clearly state.