Rotating mass is not the same as someone riding in the car. You often hear the term "unsprung" to mean it is weight that is not carried by the springs, but there are other items like cams, crankshafts, and flywheels that are rotating yet still "sprung" so its not really the same thing.
Rotating mass not only has to be accelerated forward like a passenger in the car or a battery, but also requires angular acceleration. Wheels, rotors, tires, and flywheels, even lugs can make a big difference in acceleration. In fact, the faster the car accelerates, the more energy is wasted spinning parts. There was a very good write up done on Probetalk probably 15 years ago now calculating a fairly precise measure of hp saved with a lighter flywheel in each gear. And while the rpm of the wheels are much less than the flywheel, the concept still applies.
Is it worth the cost? On a protege? Probably not. Even if it is 15hp in first gear, there are much cheaper ways to get it. $4k lightweight rims are for racing teams that need to eek out every last drop of performance.