One thing that I began to realize when I was with the JWs is that doctrine is most often formed, or changed, according to what is expedient to the organization, not necessarily as a result of serious Bible study. "What is best for the organization" seemed to be the controlling factor. When I read Raymond Franz' book, Crisis of Conscience, it became clear that my thoughts were correct.
I can remember how the JWs criticized me for studying what they called "old light." "Light" itself, as from God, is without error. Error is darkness, not light. Thus, there can be no such thing as "old light" that contradicts "new light". Consecrated Christians may be hindered in understanding the light that is given to them from the Bible due to errors, but that does not mean that any errors they have are part of the "light" of God. Once one rids oneself of an error, that error was not old light from God, it was part of the darkness that has come due to Satan's blinding influence over the world. A Christian, however, does have to grow in knowledge -- if he submits to what God has provided through his spirit in the Bible, he will be discarding the old darkness as he grows; one can grow in knowledge, however, that takes one away from the knowledge of God into the errors of man. I believe, however, that most of the consecrated never grow much in the actual word of God, for they often cannot study the Word without filtering it through the traditions of man, and thus remain as babes in Christ all of their lives.