That's exactly what a hybrid does - store energy to use when starting from a stop and at low speeds.
Hybrids aren't some mysterious magic - you can duplicate much of the same MPG results yourself by driving a tiny, low powered car, shutting off the motor at stop lights and accelerating very gently.
If you took a 46mpg Prius, took out the (hybrid) battery and electric motor and gave it to the average driver, they would still get 46mpg, maybe more with the reduced weight. It's a gimmick to get people to buy tiny cars that they otherwise would not consider.