Knowing when to take control away from the student is helpful. The instructors comfort level is key here.
It is fine to take control from a student who is 100% in control if the instructer is not 100% comfortable.
An example would be , "I took it because it was getting out of MY visual range". I agree, always let the student know who HAS IT and Why you took it.
When the student knows the instructor has limits and that the instructor is going to keep the plane within his own limits that lets the student relax a bit.