We need athletic ability. Conformation faults often affect movement. Not all horses bred should be the Olympic caliber horse geared towards the professional, the vast majority of the market is the amateur rider who had different needs. Breeding horses for different tasks is one of the elements that leads to genetic diversity. Different tastes in horse breeders also enhanced genetic diversity.
Genetic diversity is a good quality.
Interestingly, the ISR conducted a breeding experiment a few years back that catered to the large Arab mare population in Arizona. A stallion was chosen, and Arab mare owners were encouraged to breed their mares to this stallion. The stallion was chosen for his ties to Arab bloodlines. Ideal was also known to pass on size and substance that would be needed by Arab mares. This experiment was quite successful, producing, on the whole, better than average sporthorses.