The ability to lower one's mass is very useful, especially in an atmosphere. A 70 kg person who can fall from a great height and survive by lowering their mass to 5 kg while retaining the same volume will have an easily survivable terminal velocity. Such a person would also be extremely popular in the space program, as they could be launched into orbit relatively cheaply. They would also be good at one archetypically ninja trick in that they could walk across a nightingale floor without creating noise, due to the miniscule force they exert on the floor. They could creep across structurally weak roofs without falling through, climb up and down on incredibly light and weak lines that would not support a normal human, get rescued from the slopes of a volcano by a normal-sized eagle etc
However, much depends on whether muscles, tendons, bones etc with a reduced density are as strong as when the same tissues are at normal density.
- If tissue strength is proportional to density then the would-be ninja is no better off than a normal person in most situations. For example, if they jump upwards while at normal density and strength and then lower their density, their maximum jump height will actually be almost completely unaltered - the same mass providing inertia will be acted on by gravity proportionally.
- If tissue strength is unchanged regardless of density then this is basically a super-strong individual, although limits on human muscle speed will mean that not all activities will be created equal. For example, maximum running speed is largely limited by muscular contraction speed, even when running downhill there are limits to how quickly a person can run. Punching / kicking are in a similar situation, so there is no real advantage to decreasing mass while striking and then going to "normal" a fraction of a second before impact. High jump would be improved but not totally ridiculous for the same reason. The "ninja" would do best to give up on trying to be unseen and instead compete in the Tour de France - a trained cyclist who is only needing to propel a 5 kg body and lightweight bicycle would have an overwhelming advantage in the uphill stages and can resume their normal mass for the downhill runs.
Finally, flexibility is dependent on how far muscles, ligaments and tendons will stretch. There is no reason that magically reducing the mass of these body parts would change their elasticity. Any potential mechanism for increasing elasticity due to loss of mass would imply a loss of strength, which would offset other advantages. There is no reason why a character could not train to gain flexibility through ballet, martial arts, yoga or other means, but this would be unrelated to a magical ability to break the laws of inertia.
This issue has been explored in one or more fantasy books which I have completely forgotten the name of or I would give credit. The protagonist was able to both decrease and increase his weight or mass, for example, he was able to fall lightly from a height or pin an opponent to the ground.