Well it does say that in the ending his blade was deactivated and when the staff left range his blade came back which mean the staff would actualy need to be in range for him to control it. And this is why it tried to devour him cause he lost control over it.
http://www.dark-chronicle.co.uk/archive/reaver.php
Here is a site that basicly speak only of these game with Raziel and Kain. Here again it cleary says (in more detail then gamefaqs.com) this.
The Wraith Blade - When the physical Soul Reaver sword was destroyed in SR1, the captive spirit that had inhabited it for centuries - Raziel's soul - was released. It retained the sword's shape, and bound with the wraith Raziel as his symbotic weapon - the wraith blade. Thus Raziel was unwittingly carrying a future incarnation of his own soul.
The wraith blade was weak, and functioned simply as an effective weapon in SR1. However, in SR2, during
The Reaver Convergence, the wraith blade gained in power, becoming a
parasite, competing for
control against Raziel. It began devouring the souls of its victims again, and Raziel had to keep its hunger under control, or it would
turn on him.
Raziel was able to enhance the wraith blade in elemental forges, designed by the winged ancients.
What this tell me is that if the blade isnt fed by souls of enemy the blade will feed on raziel himself.
Also the parasite trying to take control against Raziel cannot make him someone that can concentrate very good. His soul of the future fighting his soul of the present.