Just a totally unsubstantiated opinion here, but I think most folks have to balance the strengths and weaknesses of their particular horse. Not everyone has the resources to raise, train and/or buy many, many horses and pick out the stellar ones who have the ability to stay calm and focused, yet still have the fire, drive and heart to go 100 miles at a rapid pace, and the physical ability to do so and stay sound. Just my opinion, but I think that in the balance, top competitors are willing to put up with "temperament" if the horse can perform at the desired level in spite of it. "Temperament" can be managed. Lack of physical ability to perform at the needed level cannot.
In our sport, we really should be looking for horses that have at least some aptitude to tolerate chaos--the more mentally-fragile ones present a danger to their riders and to those around them if things go wrong. At levels of competition like the WEC, we really should be seeing horses that can handle the stress of the chaos, since the inability to handle it not only reflects in outbursts but also in metabolic ways. (Think cardiovascular stress from high adrenalline levels, ulcers, failure to eat and/or drink, etc.--they all impact the horse's well-being as well as his performance.) The horse who can "take" the start in stride is also more apt to get through the ride in good shape!
