We never read it in the questline. It is never used. At all. Its only used in the main questline of the main game. Now, while I am glad this allows you to get the "Discerning The Transmundane" quest without doing the main questline, it would have been nice if there was an option to say, if you already read the Elder Scroll (Dragon) that "I have already read it, it has nothing to do with the prophecy" or something like that. So you dont have to rebuy the Elder Scroll (Dragon) from Urag gro-Shub.