I think there is nothing wrong with doing bounces with a young horse who is progressing nicely in its jumping. But I probably wouldn't do a ton of them (like my favorite bounce-bounce-bounce-bounce-bounce gymnastic), as they are hard work. But a bounce in a simple gymnastic, or a single bounce on its own...no problem. They can be VERY educational.
I did a clinic with Jessica Phoenix on my friend's 3 year old OTTB and she had us doing a 4 bounce line (the fences were small--alternating cross rail/vertical). She incorporated it into every course/exercise we did. She said she does it with young horses because its a safe way of teaching them how to use their feet before they progress to courses and cross country. I certainly felt the benefits of the exercise. We had plenty of messy moments but I really felt the horse pay attention and improve every time we did the exercise.
All our younf horses do gymnastics and bounces first.
It allows them the oportunity of learning where there feet are, placing them in ideal spots, and letting the rider leave them alone. I dont advocate jumping anything under four more then once a week. I go by age rule. 3-three rides week, 4-4. 5 they are pretty grown and put them into reg work.
Babies get hurt, develop bad habits much easier in course work.