Yes, the Law of God in the Commandments should continually be preached and taught in Christian congregations. But, why?
The change in our Lord’s appearance at the Transfiguration was for our benefit, that we might see just a bit of the glory to be revealed to us at the Resurrection and in the new creation. The shining face of Jesus also reminds us that He is the one who speaks God’s Word to us.
As with Moses, Jesus doesn’t send us out adrift; He continues to reveal His glory to us and be with us in His Holy Word and Sacrament.