In my career, the way horses have been worked has changed significantly. One trainer I worked for would only work a horse in a circle if it had a bitting rig on (to keep them more straight), or lines and only at a walk or very slow trot. Faster trotting or cantering increases the need to lean going in a circle

Lots of trainers and riders regularly include longeing in their program. Many trainers in other types of show horses allow “free longeing” to occur which is basically just turned loose in a paddock. Risky business for the soundness of these equine athletes. Why? 

Most of you are also horse owners, so you know that when you turn a horse loose, it will generally run straight away from you until it is a little distance, then turn around and look at you. I don’t know what predator in my country is capable of running down a horse, but that is a general procedure for an animal that is running away from something. Running in a circle is generally not a method to avoid danger.

Going in a circle may not be the best training or exercise procedure for an animal, especially a horse weighing 1,000 pounds or more. The horse is always changing direction and can become rapidly out of control.