As for desensitization itself, I find it very important. People have to remember that these horses are a LOT bigger than we are (well, most are anyway) and are all instinct. On the ground especially, if a horse spooks and shies hard at some object, let's use the dreaded plastic bag, they are going to react on instinct and won't necessarily first try to figure out where you are. They may not mean to run over you by thought but they may not be able to help it on instinct.
Desensitizing to the normal things they are going to see and/or hear, tarps, kids, bicycles, dogs, other animals, weed whackers, lawnmowers, tractors...is going to make it much more safe for both the handler and the horse. Even without a handler nearby, if a horse reacts on instinct to some simple stimulation and the circumstances are right, they could injure themselves..or worse.