If inertia manipulation really is so easy, then any kinetic attack would be futile - or alternately the technology would make the attack unstoppable, by giving the incoming projectile the kinetic energy of a small moon at 99% lightspeed.
I've been saying for years that that's basically what's happening with a photon torpedo. A matter-antimatter detonation isn't much better than a nuclear warhead in space, and starships shouldn't have much difficulty shrugging those off. But if you trigger a small forcefield that encloses a football-sized chunk of the hull and propels it forward at the speed of light, you can do quite a bit more damage than just a blast of heat and radiation (that will happen too, but not before a blast of molten metal goes careening through the ship, destroying everything in its math).
So if you have a rifle that fires tiny photon torpedoes, a forcefield or ballistic protection is going to have pretty limited usefulness. But not everyone is going to have access to photonic bullets; and even if they do, a personnel shield could be expected to stop at least one or two shots before it overloads.
Here we face the odd issue of forcefields never being up as a default, though. There seems to be little reason for starships to fly with shields down, yet they always leave the raising of shields to the very last moment. Perhaps the power required to maintain a shield is not constant or even linear with time, then, but suffers from an effect that makes it imperative that shields be kept down for as long as possible?
More likely it's just a waste of energy. Realistically, there shouldn't be any reason why shields would "weaken" when they take a hit unless the extra load causes heating of the field elements and the coolant systems that service the shields have a finite heat capacity. IOW: "Shields at 90%" is another way of saying "heat sinks are up to 10% capacity." They actually used to do this in TOS, come to think of it; channeling too much power into the phasers or deflectors for too long caused the engines to overheat, resulting in damage. In this case, keeping the shields up all the time would keep the generators running hot for long periods of time and would cause them to wear out more often, forcing higher maintenance cycles and replacements and possibly compromising the integrity of the system itself. If you keep the shields up all the time, Scotty will start complaining about having to swap transducers three times a week that are supposed to last for months at a time and begging the Captain to lay off the shields for a while.