Drill isn't such a bad concept but it gets a bad rap in the natural horse training industry. "Don't drill you horse!" Have you ever heard that?
But you shouldn't be afraid of this word when it's defined properly. Horses need consistency to learn concepts. Do something once, and they won't remember it. Do something 100 times, and they will be sure to remember it. Do it with finesse, rewards, kindness, firmness, and patience, and you will see how he doesn't just remember it, he enjoys it.
Horses don't like too much variety when it comes to learning. They might enjoy variety when it comes to casual riding or play, but not complex riding. For that, they need consistency. Try teaching a horse to not be afraid of dragging a tarp... Sure you can get results in one day, but how well do those results stick? What happens when you're out riding one day and your horse encounters a plastic bag blowing across the field? Will your horse, who's only experienced with one day of tarp training be able to handle it? Not likely. But the horse that has ten days, or thirty days of training will not blink at it. How do you get to that kind of confidence and understanding except by drilling it?
Sure, some people think drilling is bad, but only because they define drilling as negative, slave driven work. It's not. You didn't learn your ABC's by slave labor. You learned them with songs and play, through fun drills. Your horse can have the same fun learning experience. You just have to learn to value drills and repetitive exercises. When you do start valuing those drill type exercises you will become more masterful with horses.