I think that every horse should be trained to wear a bit and THEN migrated to a bitless bridle simply because there is a clearer communication with a bit imo.
I completely agree with everything you say. Except the above. I even agree horses should be taught to understand bits, simply because in another home a bit might not be an option for the rider and the horse should be educated accordingly to ensure he has a solid future. I don't believe bits are more clear communicators though.
In training my mare I use her flat halter first, her bit second, the nurtural 3rd. I set up the situation the same in all 3 situations. I put the halter/bridle on, with the reins run through the rings of the surcingle and I stood off the to side - so she was only listening to the bridle, nothing my seat or body language could convey. In the flat halter it took 5 minutes for her to understand turn her head in the direction of the pull. In 10 minutes she was turning her head the moment the slack was taken out of the rein, before it actually hit the nose piece of her halter.
In the bitted bridle (I tried a double jointed snaffle and a mullen mouth). The double jointed snaffle was confusing for her, she spent more time playing with it and fussing with where to hold it in her mouth than even thinking about what the pressure meant. Even though I had given her ample amount of time to just wear the bitted bridle without anything messing with it. By the end of the half hour she was only giving to the pressure at strong amounts or gentle nagging (squeeze/release again and again). With the mullen mouth she didn't fuss, she gave to the pressure immediately but she never went on to listening for the "pre-cue" of the rein lifting up, she always waited until pressure was on to listen. But when she did turn she was over exaggerated, in a sort of panicked turn.
In the nurtural it was just like the halter, she immediately figured out to turn her head in the correct direction and would respond before the rein ever made full contact.
My deduction from this experiment is that the double jointed bit was too "noisy" and distracting, the mullen mouth was also too "loud" for her but she understood it better. The flat halter and nutural were much more clear without being overwhelming.
But this is a horse who had nothing in her past of bits or bridles and had good teeth and a sensitive nose.
I still use my double jointed bit on my Belgian who loves the complexity of it's cues, allowing me to cue him more subtly, unlike the kimberwick which he was over-dramatic with. In a flat halter he's just a butt and will break for grass, it ends up in a fight and not fun for either of us. The Nurtural bitless gave me the same feel as the double jointed bit - I had subtle cues and good control. Because the nutural interacts with their whole face, not just their nose I can escalate my cues to mean many different things - giving me the ability to fine tune his cues.
But his roughed up nose and playful attitude I still play it safe with a bit on trails and bitless at home.