Market Harborough’s are used at our local riding school. They are primarily used to protect the horse from beginner hands. Of course the best way is to teach riders on the lunge with no reins but this is the next best thing and these horses can be used for beginner riders for one lesson snd the reins unclipped for a more advanced rider in the next class.
Draw reins I have used twice in my life. The first time was on my first horse. We were learning contact together. He didn’t quite understand. He ended up giving but would give and not keep the contact. Rider error of course but I was just learning as well.
We put the draw reins on for 5 minutes. That was all that was needed. It brought us together we both came to an understanding from it and we continued to go the same without the draw rein afterwards. I think that shows how powerful they can be, but also they were introduced for those 5 minutes at a time that both the horse and rider were ready for them in their training.
Fast forward a decade or 2 and I was having a lesson on a Grand Prix Dressage horse. We normally rode him in a double bridle, but one time I had a lesson on him and he was in a snaffle and I rode in draw reins with my normal reins. Giving with the draw reins the same as you would the curb rein on the double bridle. He went well in them.
The SRS only uses side reins for lunging. I follow their lead and that is all I use now with a loose ring snaffle and no noseband. I don’t have a horse trained highly enough for a double bridle at the moment. Maybe they will get there one day…