A horse that is dead to the rein is often dead to leg and seat aids, too. I believe the rein has a clear place in making a good riding horse . It can be soft, but ONLY after the hrose has shown a willingness to soften to it. You can't make a hors that is hard in the mouth soft by only being soft to him. I mean, if you apply soft pressure and he leans on it, and leans on it, and leans on it, but he complies like a barge loaded with bricks, and you accept and reward that with a release, you are not making him soft in the mouth, nor are you making him responsive to the seat.
You make him soft by teaching him NOT to lean on the rein, and that comes from making leaning on the rein unsuccessful, but giving to the rein, successful. You might have to get really strong and hard, at first, to convince the hrose to give .
And staying in the place where he complies to softish hand, but is stiff and leaning on the rein in't helping anything either.
Most of us riders stay in th "gray" with our horses , 99% of the time. We are not crystal clear, do not require a clean response, every time. We get "good enough and go from there. I am just as guilty. But, I won't offer soft hands to a pony who takes the rein out of mine.
Yanking is another matter, however. Just about any uptake of the reins should be measured and even, in a match with the movement of the horse (mathcing his speed/gait). Slide your hand down the rein, pick the rein up. 1, 2, 3, 4. In a meaasured way . Yanking destroys any possibility to feel when the horse is thinking about giving and has a "try " to offer you. It is NOT fair and makes a resentful horse.
This is a very good post, particularly the reference to riding in a 'grey area'. In this 'grey area', the horse goes alround ok. Everything is just ok. When I ride and train a horse, I don't want the response to my aids to be 'yeah... ok.... I'll get to it'. I want 'Yes of course! How much would you like?'.
Sometimes we do need to increase the pressure to get a response. Always beginning with the lightest possible aid of course, as this is what we want the horse to respond to. The horse isn't 'dead' in the mouth, it has learnt over time to ignore the sensation in its mouth. His previous rider probably rode with a constant backwards, 'dead' contant, not necessarily yanking on his mouth. If the horse receives no give from pressure when it gives a response, it will learn to ignore the pressure.
In re-educating a horse's mouth, we are teaching the horse that it has to react to pressure. Ask with a pinky finger at first, and gradually build up on the pressure. Yes, at times, the pressure might be significant, but if you give before your horse gives, you've taught him to ignore the rein again.
I now expect my horse to give its jaw and poll when I take up some rein. I expect lightness, I don't want a horse constantly pulling on my hands and expecting me to hold him up.
You can ask lightly all you want, but if the horse doesn't give to it, you're just reinforcing that behaviour.
My thought, is that it is far kinder for a horse to have one or two sharp 'wake up' calls, and spend the rest of its time then knowing exactly what you expect from it and how to react, than having a constant drag on the rein, or the leg etc. That is frustrating for the horse, AND the rider.