Here's the gist of it:
1. Horses may jump a bit in the wild, but they are not built to jump. CATS are made to jump. Horses? Are built to eat and run away.
2. Jumping puts a tremendous amount of strain on their legs, why in the world would you want to do that to a horse joints and ligaments that are still forming?
3. Just because a horse will jump well doesn't mean it's mentally ready for it! My vet (a reiner, but has a lot of hunter/jumper clients) who is on the national board for equine behavior says that reiners (who start at 2) and hunters (who start at 3) have the most mental breakdowns then any other discipline! Does that tell you anything?
4. Use your brains and common sense people!
5. Your horses love you, love them back!