The short answer is a qualified “yes.” In the World of My Dreams they aren’t necessary. Everybody’s horse has suitable conformation, a good attitude, and no baggage. Every rider has good hands, a stable seat, the requisite strength which she applies only as necessary, and all the training required to communicate effectively with her horse.
So wake me when all that happens. Until then, my answer goes like this:
First, here are the improper uses of auxiliary aids: 1. When force is applied as a substitute for education, or 2. When a rider cruises blithely along with her horse held in an artificial frame as she drills endless “upper level” exercises and pretends she is doing dressage.
On the other hand, here are some (to me) acceptable uses: 1. To protect the horse from his rider’s hands—with a kindly schoolmaster in side reins a novice can “find the horse’s back” and learn to sit much better than if she’s struggling to put him on the aids at the same time, or 2. With horses that have “issues” and need to be re-trained, an auxiliary which makes it easier for the horse to find a rounder outline and discover he’ll be rewarded by it can simplify and expedite the process.
I personally own a pair of conventional side reins, a pair of sliding side reins, a pair of draw reins, and a chambon. I can’t honestly remember when last I’ve used some of those devices, but I don’t view them as travesties. If they can help out and you are aware of and avoid the “razor in the monkey’s hand” syndrome, auxiliary aids do, in fact, have a place.