 The problem is that messages communicated in this way are only secret if Alice and Bob use a different one-time pad for each message. If they use the same one-time pad for several messages, Eve could gain some information about the correspondence between letters of the alphabet and subsequences of bits in the key by relating statistical features of the messages to the way words are composed of letters. To share a new key they would have to rely on trusted couriers or some similar method to distribute the key. There is no way to guarantee the security of the key distribution procedure in a classical world.