I watched a horse that was picketed by his halter completely hogtie himself with his nose to his legs and knocked himself completely off his feet in seconds, resulting in a nasty rope burn and a strained hip that took months to heal, all because he wasn't watching the rope while he was stuffing his face. And luckily his owner was there - she ended up having to cut the rope loose, he had it pulled so tightly. Not worth the risk, IMO. If you have to be there to watch them while picketed anyway, you might as well just hand graze them and not tie them out.
"Keep a leg on each side and your mind in the middle"