I agree with the post above ^. It's pretty much impossible to make generalizations like this in anything horse-related, including riding. I don't think it's a great idea for most people, but you can't just say "no one should jump before they canter" (I know no one's saying it like that) because you have to tailor each and every riding-related experience to that particular horse and rider. It's like saying unless you've been riding for 6 months and can trot with no stirrups, you shouldn't be cantering. It just depends on the person, and it's up to their instructor to decide what's best for them. To sum up this long post, I don't think it's a great idea for most riders, but I can see how in some cases it could be beneficial :).