Kant's position is surprisingly weak. Kant himself had already demolished most of the traditional proofs of God's existence in his Critique of Pure Reason . He admitted that there might seem to be a logical need for a necessary being, but the need in itself could not prove the existence of such a being. Kant did not use the same critical approach on his own ethical argument. Proving that God might come in handy for an ethical system is not the same as proving that He really exists.
Religion provides ethical certainties.
Religious sanctions are needed to back up ethics. Religion can only provide a solid base for ethics if religion's own base is solid - if it is founded on irrefutable evidence or on common sense that is plain to all. Yet there is no evidence outside scripture for the existence of a judging God or of a painstaking process of allocating new incarnations on the basis of past deeds. Many believers retain persistent doubts, and these doubts would give them the leeway to commit sins. There is no philosophical reason to obey God.