A horse will need extensive work before even considering doing this.
You do NOT picket a horse by it's halter. That is asking for a hoof through a halter. You picket by a singular hobble on a front leg.
It is a wise idea to hobble train a horse prior to picket training, as it will be a good foundation.
One reason to picket a horse over simply allowing a horse loose while hobbled is that some horses will learn to move very quickly in hobbles.
It is a matter of preference for the most part, but done correctly with a horse that has a good mind for it, there is nothing wrong with supervised picketing.
Proud owner of ~Mana: 6yo Arabian gelding~Pearl 13yo Arabian~Danzer 14yo Arabian mare~ Tiny mini filly