"I totally agree with "leave alone if horse gets worse with trimming" as duly taught by Kingsley.
Just a random thought - seeing hooves are always changing in response to work the horse does...and seeing the shape and sensitivity is dynamic and changes in response to biomechanics and diet etc....
Seeing all that, I wonder if leaving hooves without shoes AND corrective trimming would make us all pay more attention to how we school/train/compete our horses.
If, let's say, schooling/dressage a horse with weak back muscle at a level it is not ready for, in hollow outline and by a rider with underdeveloped seat would cause the horse degree of back pain that would quickly lead to more limb strain that would then lead to sensitivity in bare feet/lameness then perhaps the whole training would become wiser.
As it is, so many issues in the horses' bodies that are the results of uneducated riding can go on and on for a year or longer. These issues are "treated" and "cured" for a moment only to resurface with greater force and show up as lameness.
Perhaps having horses barefoot and largely self-trimming, a horse that shows any issues in weeks or days rather than years, we would grow more sensitive to training the whole horse and be more tune in to their needs and body issues."
This is sentiment I completely agree with and for me its one of the biggest benefits - and biggest trials - of having a barefoot horse: it is much, much more difficult to overlook a problem when a horse is barefoot.
This is true whether the issue is lack of training or a biomechanical issue, as Wiola's post demonstrates, or whether its a more general health concern like a nutritional imbalance or metabolic problem.
In a lecture I went to last year, Prof. Jean Marie Denoix (talking about shod horses) made the point that a "sudden" injury or lameness is rarely that"; in fact, the precursor of most injuries is months or even years of fatigue and progressive stress on tendons. As he said: "a tendon injury is rarely a primary cause" - and if you don't identify and solve the source of the injury, it will inevitably recur.
But imagine if you had an incredibly sensitive early warning system which could detect and alert you to these early signs of stress, fatigue or malfunction. In a barefoot horse, this is exactly what you get.
Hooves are an unbelievably accurate barometer of the overall health and fitness of the horse and will usually provide early warning of potential problems which can often then be nipped in the bud - dietary changes can be made, overworked soft tissue can be rested, weaknesses strengthened.
However, in many shod horses, this barometer simply can't be read because the hooves are no longer capable of providing subtle feedback - wear patterns cannot be seen, proprioception is reduced and, even if limb-loading changes, deviations in the hoof capsule are removed by trimming and shoeing.
The level of detailed information a bare hoof provides can be uncomfortably immediate and uncompromising, but on the other hand, if you have a very sound, robust hoof then you can be pretty confident that you have a sound, robust horse.