Throughout the series different half-beings need to do something to become fully what they are. Red Court need to kill someone, White Court need to kill someone, some of the Fae need to make the decision of either being mortal, or embracing the Fae side.
I'm wondering if there is something similar for Harry, and that's why it's been kept from him. They know that the desire for more power may be too great, and if he had the knowledge of how to activate his abilities he'd do it. I think him wanting to save his daughter is an example that he'd do whatever it took to keep her safe, and becoming a another Drakul wouldn't be too big a price.