For years, police departments have prohibited their officers from using warning shots to effect an arrest. In addition, no defensive trainer worth the title recommends the use of warning shots. In spite of that, each year a few citizens are involved in the use of warning shots, generally with disastrous results. I suspect most of these folks are people who have not sought professional training and have not developed a personal-defense plan.
In many cases, the armed citizen is faced with a situation that could become deadly, but has not yet reached that point. They pull their gun and are amazed when the bad guys don't run for their lives. Instead, the bad guys laugh and taunt them. Now the frustrated citizen feels he has to do something to let the crooks know that he is serious, so he cranks off a round or two. The results of such a hasty move are generally all bad.
To begin with, the armed citizen has just depleted his ammo supply. If things suddenly get worse, he may not have all of the ammunition he needs to resolve the issue in his favor. Unless the person is one of those Tactards who thinks he needs to carry a dozen magazines at all times, he may have just placed himself at a tactical disadvantage.
But the most serious problem with warning shots is that the courts will hold a person responsible for every shot he fires. In short, you own those warning shots. If they impact innocent people or property, the courts will not accept the claim that it was accidental. Not long ago, a hunter fired his muzzle loader into the air to unload the rifle. The bullet traveled some distance before it came down and killed a young girl. You can bet that the incident was treated as negligence, not an accident. Though this was not a defensive situation, it illustrates the fact that a person owns every shot that they fire, for whatever reason.
The only justification for firing a defensive weapon is to counter an immediate deadly threat. If the threat is truly immediate, then the shot should be fired to the vital zone of the attacker. The courts will probably suggest that, if you had time to fire warning shots, then you had time to seek other, non-lethal, solutions to the problem.
The serious defensive shooter seeks proper training on how to handle deadly confrontations and he develops a personal defense plan for dealing with those confrontations. Warning shots waste ammunition, endanger non-combatants and create unnecessary legal problems. That's why I say warning shots are pure poison.