I've run across quite a few people that believe that no horse should ever be ridden using a curb bit, ever.
The argument advanced is that the horse needs training to be able to be ridden, and that a curb is covering up the problem period. Quite vehement too.
I prefer a curb bit, always have. My argument is that a horses does know what it is being ridden in, and may decide to take advantage of that, and all the training in the world, won't overcome a willful horse that decides to act up.
So what say you?