There is a lot of information out there about correct vs. incorrect training of the "modern" purpose bred dressage horse. All you have to do to see both examples is look at any high level dressage competition or sales videos. You'll see a lot more incorrect than correct. But what about correct vs. incorrect training of the not purpose bred dressage horse? You know, OTTBs, Quarter Horses, and Arabians etc..
This is a pretty good example of a non-traditional dressage horse going incorrectly and correctly. You can see how when the non-traditional horse is ridden/trained incorrectly it is very obvious because of the smaller gaits. Much more so than with the purpose bred dressage horse. Now compare the two illustrations. Do you see why incorrect purpose bred can beat correct non-traditional in the competition ring? Do you see that in training and riding the "modern" dressage type with fantastic gaits, cadence, rhythm, and penchant for collection, it would be pretty easy to get 60%ish scores while still not being correct? Do you also see how even with the improvements in strength, cadence and stride, the non-traditional dressage horse is correct and trained classically but that horse will still not have as good of a score as the "modern" dressage type going slightly incorrectly?
This is why people who are serious about competing buy a purpose bred dressage horse. There is a huge built in competive advantage. However, I think a lot of those riders score in the 60% range, climb up the levels, but still feel dissatisfied because their biomechanics and their horse's biomechanics are not quite correct. Lots of slightly behind the vertical, wringing tails, dropped backs on horses that don't look quite happy. Riders too. On the other hand there is no faking it with the non-purpose bred horse, so the training MUST be classical. The ultimate scores will never be as high as with the purpose bred horse trained correctly, but the satisfaction of turning a duckling into a swan is pretty awesome as well. Thoughts?