There has been much conjecture over the bars and what to trim and how to trim etc etc.
I don't have all of the answers but I can point out some observations about the bars and how they should look on the average hoof.
First of all consider that the bars are hoof wall grown in next to the frog much the same as other animals have a nail around the pad, they are essentially the same structure and it should have the same exfoliation process.
The sign of a healthy hoof is when the bars exfoliate just like the rest of the walls. The tubuals become exposed and separated from their keratinising compounds and basically breakaway a little above the sole. If this is the norm for a healthy hoof what does that mean when we see all of the problems with the bars.
How do you know you are headed toward a healthy hoof and healthy bars. The first thing I expect to see in a good foot is a front half of the sole with a polished leathery look rather than the unhealthy white flakes you see in shod hooves. In the transition stage you see these polished soles about 3 months in and then by 6 months the back half of the foot is the same as the front half and the bars are nice little lines that look like hoof walls in the back of the foot. The exfoliation of the sole and bars is so fine it does not appear to happen, much like out own skin we know it exfoliates all day every day but we never see the process.
How do you get the bars to act like hoof wall, well you have to chase them and chase them to get them to conform to being just like hoof walls.
I have seen comments that say that the bars are supposed to extend towards the tip of the frog and that the lumpiness in the sole along side the bars is normal. How can this be true when a healthy hoof has bars that exfoliate like the walls do and the sole is perfectly normal from end to end.
These lumpy out of shape bars and distorted sole nearby need to be trimmed back to keep them from interfering with the health of the hoof. If they are left on the sole you can find bleeding into the sole tissue under them, so while they may desensitise the sole to the damage going on by restricting circulation a bit like a horse shoe does it is not a good idea to leave them there. There are subtle problems that arise from leaving these lumps in the soles. The key to hoof health is circulation something like a lump left in the sole near the frog can lead to inflammation and this inflammation can lead to pressure and rerouting of blood flow to the solar corium. This may lead to boney changes to the pedal bone or new foramens appearing to compensate for the trimmers lack of understanding in trimming the hoof, It is the unseen damage that is the problem with not trimming out the over exuberant growth of sole tissue. The reason the tissue appears is that the corium is damaged due to excess pressure when it was incorrect the body then feels the lack of pressure and grows faster to fill in what it feels is a unwarranted gap, the problem is that it takes time for the corium to realise that it is no longer in panic mode. Once the corium heals the exuberant growth will slow down, if the excess tissue is not trimmed away the impact can be far reaching, shelly crumbly feet is a symptom of inflammation within the hoof capsule disturbing circulation, you end up damaging not just the area under the excess growth but the whole hoof can be affected by the lack of trimming in these areas.
We have so much to learn about the hoof even still, but being observant of things beyond the localised problem is key to achieving the best hoof for the horse. If you leave sole alongside the frog you may just be damaging the pedal bone and the wall density.
It is a disservice to the horse to impose our will on what the hoof might or should be, those people that feel the bars should look like something they are not should look at a healthy hoof and see how the bars live up to their structure and design.
The biggest problems with hoof rehab occur is if the bars over lay the soles or worse impact into the sole, I have an amazing photo of a hoof with overlaid bars to the point there is no true sole left the bars have impacted deep in to the sole and crushed the solar corium. The only way to grow new sole is to relieve the pressure by reducing the contact the bars have with the ground and not overly thinning them. You must consider a bent bar as exactly the same as a wall flare, they both are walls and they are both bent out of position. So if we realise that a flared wall is a bad thing and we trim it back to fix a problem, why do some not trim bars like they are flares. The disturbance to circulation should be enough let alone hoof balance and corium damage.
The bars are quite often a source of abscessing and damage within the hoof and efforts should be made to trim the bars to behave like they should. I once saw a barefoot horse that had been lame for 12 months and the only thing wrong was impacted bars, I only had to trim 3mm in height off the bars for about 1 inch but the pressure relief was life changing, here was a horse confined to 12 months rest under vet orders due to an unknown hip problem that was fixed and sound in 2 weeks by the correct trimming of the bars.
Bars when trimmed out of horses with excess length can return within two days, this far exceeds the ability of cell growth to replace the trimmed material. This indicates that the bars corium is affected by the pressure and is returning to a normal position leaving more bar exposed. It is not the horse healing damage done by the trimmer, the body has no way to produce that much material in 2 days. I have seen feet that have a dry weak looking wall and sole, trimming the foot correctly fixed the circulation issues within the hoof and it became functional and strong. Don't just consider the bars and excess sole around the frog as a support structure when it is possibly impacting on the rest of the hoof in many ways.
Once again take your eyes off the hooves and look at the upper body it will tell you if you are getting the bars wrong, little muscle tremors indicating the horse can't cope with the extra unnecessary pressure created by untrimmed bars can make a horse uncomfortable or a cripple. Just picking up a hind leg that seems sound may show little muscle tremors up near the thighs due to pain from returning circulation.
Remember the key to a good hoof is circulation you have to understand what solar pressures will do to the hoof's circulation, if you get one part of the hoof wrong another part may try and compensate. The key is to reduce pressure and allow the true hoof to appear.