You may have heard or been exposed to the term stabilization when exercising. Stabilization means having well orchestrated control from all sides of a joint. Yes it's incredibly important and required before you add more resistance to an exercise. However, sliding a wobbly ball underneath your feet or standing one leg is not the answer. Adding instability to an unstable body only leads more instability. Will you get better at standing on a ball if you do it a lot? Sure. Your skill will improve like anything you practice more, but don't confuse gross external control for improved internal joint management.