Actually - now that AQHA is recognizing the discipline and giving points for it, I am very sure you will see more and more stock horses - quarters and paints - excelling in the dressage ring. The leggy types will do very well.
Dressage is excellent cross training for any discipline.
What worries me is that just as everyone goes out and buys the dog the wins Westminister to be in fashion, that people will be buying off breeds expecting them all to be the next dressage master.
I was certainly not blinded that I had the "odd horse out" with my quarab and the funny thing is Arabs were considered a no no until I got to FEI then it seemed like everyone thought they were the best choice (and cheap).
I remember judging one arab show ( I think I was chosen because I was "arab friendly....LOL) and gave 2s and 3 for one horse that did a semi false passage in place of a working trot at the basic level.
It is this "in fashion" type of following that we cannot allow those that have off breeds that will never do better than the basic levels to think that because one atypical individual having the best training excells.