"Dressage is for the horse, not the horse for dressage." That's one of the classical guidelines. Dressage should make a horse healthier, happier, more beautiful and confident; and extend his useful riding years. But does it? You have extreme views on one end, pretty much arguing that anything other than putting a horse out in a herd in a field is abuse, and on the other end of the spectrum, "training methods" such as "Rollkur" (there is nothing "curing" about it) are considered acceptable. Add to it that good riding (in any discipline) is not easy, and frustration more common than patience and skill, and at some point you have to ask yourself: Is my sport doing right by the horses, and am I creating healthier, happier partners? And can dressage accomplish this even in the best of scenarios?
I always find it interesting to look at a horse's muscling (or lack thereof) when I start working with them. An older gelding came to us with no discernible topline, his long neck sticking straight out. Within a couple of months of training (and appropriate veterinary care to deal with his arthritis and teeth), he started building proper muscle and is quite happy now to show off some of the tricks he had learned way back when. He still gets a little excited about his tempis, but when he's truly on my seat and over the back, I let him indulge here and there. His pride is palpable. He's also looking quite handsome now, much to the enjoyment of his owner.
A very cute 17 yo Anglo Arab mare I work with had had a bad reaction to vaccinations, possibly combined with an injury, that very much disabled her. It took her dedicated owner a couple of years of veterinary diagnosis and treatments (including chiropractic and massage) to restore basic riding soundness. But she still did not develop any muscling over her back. After I started working with her, I thought I could detect a hint of topline, but I wasn't entirely sure. She needed the slow therapeutic riding that Andrew Murphy and Charles de Kunffy teach. The patience paid off and she started developing the right muscles. Always checking in with her vet and bodyworkers to keep everyone on the same page, I got confirmation that what I was feeling was also manifesting in her body: she got stronger and with that more confident in her body.
This winter, about eight months into the work, the mare had a hoof abscess (as did almost every horse in Northern CA with all that rain), and was off for about six weeks. Her body did not do well with the break and she was still quite lame despite the abscess having cleared. I feared a serious injury, but she was examined and received body treatments-- and I was told to get back on. I was skeptical to say the least, and also disappointed as she is such a sweet and willing horse. Within the first ride, just walking of course, I could feel her body letting go of tension. By ride three she could trot a few sound circles. Her owner said she looked like she was smiling the whole time. I would have never believed that a horse looking that off on the lunge will improve this dramatically by therapeutic riding in just a few days. (Disclaimer: this horse was cleared to ride by veterinary professionals because of her particular circumstances, the lameness was due to tightness, not actual injury, I obviously do not advocate or believe in riding lame horses...) This mare was able to bounce back quickly this time because she had worked properly and gotten so much stronger over the previous months.
That is exactly what dressage is supposed to do. It is proof that in conjunction with good veterinary care and general management, good riding, i.e. dressage, is indeed an active ingredient in creating and maintaining healthy, enthusiastic, productive horses. And what could be a better profession than to make that happen!