Dick Kimball, the 1964 U.S. Olympic Diving Coach, posted a .PDF article on use of spotting rigs for divers. Kimball likes them, especially for age group and developing divers.
Better, I think, is for diving coaches to consult with expert trampoline coaches who use progression instead of belts. I would only use a belt if I had problems with the progressions.
Coaching gymnasts I avoid belts if possible, except for very few specific gymnastics drills.
They can, however, be a great novelty for recreation gymnasts. I like using a spotting rig for carrying beginners through backward handspring, for example.
Gymnastics Coaching.com 