I don't have feelings about people teaching their horses to do tricks, one way or the other. It's their horse, not mine. When I was younger I had a horse that knew how to rear on command, lie down, and bow. Horses that are trained by good trainers should not do anything dangerous and not on cue. And if you are not a trainer or a good rider, don't train or ask your horse to do anything you can't do. Your horse could jump a 6 foot fence, but if you are not an experienced rider you shouldn't be riding him over 6 ft fences.
That said, I don't see any reason to teach a horse to rear because it can be a dangerous habit for anyone other then a very very experienced rider/trainer and is not useful for most horses and riders. I don't know how to train a horse to rear on command so I would never do it. So I guess, unless you train with the Spanish Riding School in Vienna, work for Barnum and Bailey, come from a circus family of horse trainers, or are a rider at Medival Times, you probably are not qualified to teach your horse to rear properly and you shouldn't do it (well, do it if you want but good luck). :)
As for the freaky video....I don't understand why someone would take the time to make such a thing? What's the point? I doubt that all those horses in that video were not rearing on command or taught to rear but rearing from fear, flight, or fight.