Apple Cider Vinegar is great for abscesses. Excess moisture IS bad for hooves, but in the case of an abscess, it can soften hoof tissue enough to let the abscess pop sooner rather than later, and it's acidity helps to neutralize the body funk that tends to be more alkaline, thereby lower the chance of reinfection by greeblies that feed on things like pus and whatnot, so it creates a "hostile environment" for opportunistic organisms, like certain bacteria and fungi/yeast. For soaking, usually diluted to about 50% acv/50% water is adequate. So I wouldn't soak the hooves with it all the time to prevent anything, just for specific purposes.
It can help grow out hoof cracks, soaked or sprayed, depending on the situation.
To prevent problems, I use it mixed 50/50 with water in a spray bottle and spritz the frogs after cleaning about once a week. This, again, creates a hostile envrionment for thrush organisms that feed on poopy covered, shedding frog and the sweat (yes, they have sweat glands around the frog).
ACV doesn't increase hoof growth, that I've ever seen, but it IS used as a joint supplement and fly repellant (Feed-thru and spray on). If you want to feed it to your horse, I do recommend buying it from a health food store, as opposed to the grocery store brands. The grocery store kind is distilled, and works fine topically, but the "mother" or slimy part that has all the good stuff in it is removed by that process, but the health food store variety will stilll have it in.
Just a side note, ACV is bascially sour wine made from apple cider. Pretty neat, huh? All vinegar is made from sour alcohols. I did some resarch on it, and thought that a fascinating factoid.