You have to weight the advantage of the training versus the perceived risk involved. In holding the brake, I could easily have applied opposite brake (which I did). On a 150 foot wide runway, I knew that if we should veer to one side, we would A) not go very far with one brake held, and B) be able to maintain control with the opposite side. Now I was exaggerating a bit with 180 degrees... it was more like 135 degrees. But we were at an uncontrolled field without traffic in the vicinity, and I felt letting the airplane pivot (not turn, but actually pivot) would make the situation stand out in the mind of the student. If it was a tighter runway, slippery, etc., the situation would have been different.
The object of doing "emergency" and "abnormal" training with students is to get them to see some things that might happen out flying that they wouldn't otherwise see. At my school we had an excellent maintenance department and new airplanes with a high level of reliability. Therefore, the students didn't have to deal with many actual emergencies. There is a limit to what you can do, and you have to use your head about it.
As a CFI you should try and show your students as best you can some abnormal situations they would see in flight. However, you should always ensure what you are doing can be undone safely should the student not react at all, or act in the incorrect manner.
For example: What happens when you start working as an MEI? You are essentially turning off an operating engine to train for an emergency procedure. But it is necessary so that the student can grasp the severity of the situation. The same can be said for the open door. Now that you have seen what happens when a door opens in flight, are you not better prepared for the loud commotion that ensues?
You should NOT take chances as a CFI. But you should ensure your student has experienced a variety of abnormal and emergency simulations so they will not freak out someday when the not-so-well-maintained rental decides to break on them, and turning to the right to look for help they find an empty seat.