Ok so here's my quick opinion:
Kneeling exercises, such as kneeling box jumps, kneeling cleans, and kneeling snatches might provide some benefit in improving the olympic lifts. They definitely would be good at teaching the body to catch cleans/snatches in a deep squat position.
These kneeling exercises also provide a nice REA (reactive) or ADA (altitude drop) effect, since you are exploding out of the kneeling position, getting a bit airborne, and dropping into position. The kneeling box jumps are mostly of the REA form, while the kneeling snatches/cleans are in the ADA form.
However, the REA effect of a kneeling box jump just doesn't seem to intense, considering the freefall is small, and the weight to overcome (bodyweight + low KE) is small. For example, there is absolutely no comparison between a depth jump from 18+ inches and a kneeling box jump. The amount of force needed to overcome the freefall in a depth jump is much greater than that of a kneeling box jump, this leads to a greater training effect.
The ADA effect of the kneeling snatch/clean is also much lower than that experienced in a depth jump, depth drop, or REA squat. Taking this into account, with the fact that most athletes won't ever work up to any impressive poundages with these lifts, and you have a recipe for waste-of-time.
So, due to the above points I made, I conclude these exercises are a waste of time for improving SVJ/RVJ (standing vertical jump/running vertical jump). Instead, they may be beneficial for learning how to catch a clean or snatch in a deep squat position. Besides that, these exercises could be used as boredom busters or to impress women at your local crossfit pukefest.
adarq's verdict: WASTE OF TIME FOR VERT.
Poll