Stability training with such items as the BOSU Ball have very little place in training health individuals. However, they do have a place in rehabilitation.
Stability training with such devices is more of a cult. Reasoning with those individuals is impossible.
In "Training Disasters" by Tony Gentilcore, below, he explains why there is little value in using these devices with health individuals. www.t-nation.com/readArticle.do;jsessio ... id=1785278
By training on an unstable surface you're shooting yourself in the foot in a few ways.
1. You'll burn fewer calories.
You won't be able to use nearly as much weight on an unstable surface as you would on a stable surface performing the same exercise. Burning calories (and hence, fat) is all about progressive resistance/overload (stressing the body). BOSU balls and the like are inferior in this regard.
2. You will actually make yourself weaker.
Gaining strength is all about force production (i.e. being able to transfer force from the ground up). By training on an unstable surface, you're promoting "leaks" and really limiting the amount of force you can generate in any given exercise. For athletes, this is crucial.
Additionally, unstable surface training undermines all three phases of the stretch-shortening cycle (SSC),
especially the amortization phase. Using devices such as BOSU balls actually delays the amortization phase. From an athletic standpoint, one seeks to minimize this phase as much as possible to prevent loss of a significant amount of the energy accumulated and stored as a result of the preloading (eccentric) phase.
3. Core Strength? Please
don't be that person who claims that unstable surface training is great for training the core. I can have someone do the same movement on a stable surface and activate the "core" just as much.
4. Safety. On more than one occasion I've seen people fall and injure themselves while performing exercises on an unstable surface. For example, squatting on a SWISS ball. Every time I see someone attempt this, I cringe. Free-weight exercises have been proven safe when performed on stable surfaces, but there isn't much data out there which showcases the efficacy of unstable surface training in regards to safety.