I wondered the same thing sort of, in a thread I made a while back. This is what I said:
-When Kabuto creates his Edos he places the controlling talisman usually in the head of the dead right. If someone from knew he did this couldn't they just remove the head of the dead and take out the seal to make the dead have their own body back and decided for themselves who to fight with?
For example:
-In the battle with Kakashi vs Edo Zabuza
-If Kakashi was to slice of Zabuza's head with a lighting blade and after doing so find the controlling talisman and remove it
-Zabuza would still be an Edo but would also have free will to act on his own, so in turn he could help the ninja alliance.