She wants power in the organization. But they don't want it to come to light that with her lending he can take on additional powers of ALL the captains (one at a time, anyways...maybe) making him incredibly powerful and versatile.
Am I not seeing something? Because this sounds like a great way to make herself (and him, by extension) indispensible to the overall group, making it much more likely she would be the next head if the current one stepped down for whatever reason.