I have used them on a VERY specific few horses, all closely fitting Anebel's description of the dangerous horse that throws its head and bolts. 3 were off the track thoroughbreds, though I also took one QH x appy mare that was a real terror.
In all cases, the horse had absolutely no comprehension that it would in fact release its neck and balance using its body, not it's head. No amount of transitions, leg yield and changes of rein were going to get those horses relaxed any time soon, so as a very last option, the draw reins were bought out. I rode soley off the snaffle rein, handling the draw reins as though a curb rein. The only time the draw reins were touched, was when the horse would hurl its head in the air, twist its poll, lock the jaw, and run. The action of the draw reins didn't take long for the horses to all understand that they could actually be comfortable in a lower 'frame'. The most rides on one horse I ever used the draw reins on, was 3. They do NOT need to be used as the only set of reins, and they do NOT need to be used on a continuous basis. They are NOT to be used to achieve a constant 'head set' as in this case, they tend to teach the horse to suck back from the contact and lock at the wither and poll.
Just give them the idea, then go back to riding properly.