I think gymnastic jumping is invaluable in training a dressage horse. Besides the fact that it is fun, a brake from a regular work and something new for your horse to do, it is very beneficial in athletic development of a horse. The jumps should not be tall and it is a good idea to built some grid lines not just an individual jumps. Gymnastic grids are used to train horses to jump, they help a horse to take off correctly and find a correct form. And jumping over several small jumps is very good for developing your horses back and hindquarters. Another benefit is observing your horse's reactions: is he honest in his approach, apprehensive, does he begin to worry and start getting excited? That will give you a new knowledge about your horse. Tackling obstacles together will help to further develop your partnership.

If you worry about your horse getting injured do not jump higher than 2 feet. Such small jumps will not stress your horses joints more then any other dressage work. You can jump them in your dressage saddle with shortened stirrups. And the jumps are not big enough to intimidate your horse if he is not really into jumping. As with any other work proceed slowly, make it a fun day for you and your horse. You can find information on how to built grids in any book on jumping or on the Internet.

Unless your horse has unsoundness issues there is no reason not to jump him a little bit. If you do not have enough stable seat for such small jumping then you do not have enough seat for a dressage work.

Happy riding...