There are 2 aspects in a horse's jumping... 1) they rock back on their hocks and launch their body over the fence with plenty of arc and 2) they lift their legs evenly up high to clear the fence. A naturally talented horse will be able to do both, but you often see an uneducated (or nonathletic) horse do one or the other until they are taught otherwise. Gymnastics (bounces in particular) are one of the best ways to teach a horse how to properly and efficiently use its body over the fences because they don't have the forward momentum to get them over, they must learn to use their body and tuck their legs. The fishing line trick may or may not work, but to me sounds like an accident waiting to happen. Gymnastics (and cavelettis) are safe and effective and teach much more then just lifting up knees.
I wouldn't consider dangling knees as a sign of not being cut out for jumping unless it becomes dangerous or they show other signs of not being talented over fences. There are several jumper trainers I know that actually prefer a low kneed horse that has the scope to get over the fence as long as they don't touch the rails. (in fact, it's much easier to teach horse to lift his knees then to use its body) In fact, I saw a jumper last week with dangling knees jump a 4"6 course and go double clear in a class of over 30 horses.
Rare, but does happen.