A breastcollar worn for pretty isn't affecting the horses movement or ability to perform.
Nobody's arguing the fact that a too tight martingale causes problems. It's saying that martingales are eeevvviiiillll or that your horse obviously isn't trained correctly if you use one, that has people a little miffed.
They're no more evil than any other piece of tack or correctional device when used properly so that they don't interfere with the movement and performance of the animal.
Just as a Western rider wouldn't dream of going into the ring without their hat, chaps and bling, neither would a hunter rider go in without a martingale.