We aren't actually telling anything more than Tolkien did, to be honest. We still have no idea what he has been doing throughout the entire history of the universe. That he is a Maiar is a conclusion I immediately came to when I first read the Silmarillion, based on what Tolkien said about him. I didn't make that idea up. It just seemed a natural consequence of what had already been written. The question I have is: What other cool possibilities are being eliminated by specifying that Glaurung is a Maia? Could we suggest that Melkor is able to recruit outside Ainur from within Arda? Or could we suggest that he can create spirits himself? Or that he can amplify and dominate the spirits of Elves that significantly? I'm really not comfortable with any of these, and I am not sure that any of them are supported by the text in any way at all. So what are we missing? What are some other possibilities that don't break the world that would be given short shrift if we know that Glaurung is a Maia?Sure, but what does it actually ADD to the story if we make up more of a background for him than...well Tolkien himself actually did? I still don't under stand that strong drive to tell so much about Glaurung before he is finally finished..