My mare is half Shire, and gets really feathery. I've never seen a reason to -not- trim. But I don't trim them often - only once in a while so that I can see her whole hoof for a trim and make sure there's no flaring. (Obviously I'm not just counting 'feathers' as what's on the back of the lower leg and pastern, but all the way around the leg.) One thing I like about feathers in the wintertime is that they are thick enough to keep her hooves dryer - they keep moisture and mud from reaching the skin. If I wipe the mud away and brush them, the underlying layers of hair are dry, and I can clearly see clean skin underneath. Even the cuticle stays fairly dry, where it might otherwise have gotten crumbly/squishy/mushy.
In regard to the concern about scratches (fungal infection of lower leg skin): I don't believe that clipping or not will dictate whether your horse gets scratches or not, as it's not an issue with an aerobic vs anaerobic environment. It's an issue with what's in the soil and activation of infection in a damp/humid environment. Before the drought (~2006-2012), my pasture and the surrounding 7 acres (which used to be home to 7 to 9 different horses) always endured a wet, muddy winter. But scratches was unheard of. With the weather changes in the last 5-6 years, rainfall dropped 90-95%! So we have humidity and warmth, and I've noticed mushrooms popping up everywhere they can... even in areas that get a lot of sun. I didn't think much of it at the time, until 2 years ago. It was bone dry and hot outside, and my mare would not stop running around in her pasture about *something*. From what I can tell, she got excited about something, ran around in the hot weather, popped a blood vessel from doing that, smelled the blood and ran around more. I treated the nose, then hosed her down to cool off...decided to turn that into bath time. Dried her off well afterwards - and I know I dried her legs well, she's got feathers! - and sores were present 1 week later. You bet I was very surprised - I'd never actually seen scratches before, and when I learned what it was I said "Oh you've got to be kidding me!" It was literally 85 degrees outside in an open, dusty, sun-soaked pasture...and one bath set off infection. Apparently some fungus can live in heat up to ~104 degrees F.
So if your environmental conditions are supporting fungal spores, hair thickness seems to not matter much. Short hair likely makes it easier for infection to start, and feathers keep the environment dark and insulated if spores reach the skin. So better than clipping is this: Treating for scratches before scabs start, keeping the leg dry (wrap, etc.) if it looks like the weather will be bad, and keeping your horse out of the mud. To treat early, I'd stay away from bathing (via my story), but rather use medicated spray (Krudbuster, Vetericyn gel, or Bannix) to really soak the area, even a little bit above where scratches might set in. Then massage it in well, wrap the lower leg, and repeat daily for a few days, at least. You can also help bolster your horse's immune system through diet - Vitamin C.
When my mare got scratches, I also did this on the day I discovered it: Soaked 50-50 mixture of Triodine with warm water in a clear leg soaking bag, massaged the area where the scratches was, put hoof into a PVC Davis boot, and secured the bag with a ring of duct tape around the cannon. Waited about 5 minutes for each leg. (This soak is very good for hooves and is one method to help with thrush if the hoof has already been cleaned, as Triodine kill fungus as well as bacteria. It's easy on frog tissue too! If you use triodine, though, it stains.) With the hair and skin clean and disinfected, I dried her legs with a towel and immediately started a daily treat-and-wrap. For the wrap, I used a baby diaper with the elastic cut off (I had these around for hoof abscess treatment), pressed it to the treated pastern, wrapped with vetwrap, and then secured with duct tape. (Duct tape also helped keep everything dry when the rainy weather started.)
If you want to treat your soil to get rid of or reduce fungus, there are various methods to do this. Solarization is one, and is pretty safe. Fungus likes acidic soil - if your soil is acidic, you might choose to have a lime-sulfur treatment done. (I also read that lime kills weeds by depriving them of acidic soil, and also promotes healthier pasture grass.) This type of soil treatment can be harsh - it requires a person to wear protective clothing, and the horse cannot have access to that pasture until some time after a heavy rain. Despite that, it's still commonly used.
No diet, no hoof. No hoof, no horse. No horse is not an option!