What do you mean unless?
We KNOW that's the case. Madara and Tobi can both summon the Kyuubi at will, and anyone with a Sharingan can apparently mind-control them.
I think they also said at one point that the control that a Jinchuuriki has over their powers actually makes them more dangerous than when they're just running rampant.
And I think this week kind of proves that point. Normally, when a Jinchuuriki goes full beast mode, they just look like the Bijuu naturally looks. But in this latest form of Naruto's, not only is he all runed up a glowy, but Kurama is as well.
It gives the impression that their combined form is actually MORE powerful than Kurama would be if he was simply free.