I would be bold enough to say that 95% of horses on the rated circuit show in either some sort of a D ring or a pelham. They are VERY acceptable and I have never heard of anyone getting counted off, even as a tie breaker. I see them in the upper levels, I see them in derbies, I see them everywhere. I've used one on several horses for years and as far as I know haven't been judged unfairly because of it.
In the eq ring they are not only acceptable but in this day and age the latest fashion whether or not you need one. I watched the Maclay finals a year or so ago and literally EVERY SINGLE horse had a pelham. And i'm certain some of those machines didn't need one!