I don't get this idea. Because you want the horse to WANT to focus on a mundane task rather than being trained to do so, you think adding a distracting bit for him to play with will help?
I guess it would be the same philosophy as giving kids 'fidget spinners' to help them concentrate in class. I don't get/agree with that as a rule either.
Yes, that is essentially it. Since he is already inclined to play with the bit I would like to give him one that's meant to be played with. When we are standing, he can play instead of look around and get antsy.
In my experience with young children who have difficulty concentrating, having something to play with can really help them stay focused until they grow older and then that object they play with transfers to something else. In most of the classrooms I've been in, and in my own we do give children who have a hard time concentrating some form of accommodations. This could be a wiggle seat, so that if they can't hold still they can wiggle while causing less disturbance, I also use marble mazes, I've seen parents who give their kids legos to keep in their pockets to play with. It helps give them some form of stimulation so that they can stay focused on low interest activities. Usually those same kids have a subject they love where they don't need to play with something, because it's high interest for them, and they invest fully. Now if the object they are playing with is too interesting (fidget spinner), it can totally distract them more, it needs to be something small that's almost not noticeable, something appropriate for an adult might be chewing gum, or clicking a pen.
Aside from my personal experience it's what many schools do, and what the books teach here in California. I'm sure there's some studies out there supporting why we do this. Not everyone agrees and the parent has the right to say no, not for my kid... I wouldn't ever give a kid a fidget spinner, but I might give them a marble to roll in their hands while they listen to me read, for example.
I think it goes without saying this isn't any sort of rule, and it doesn't apply to most people, because most people don't have problems learning in this way.
If having a fidget spinner helped your child focus in the classroom, would you let them have it, or not?