That is a part of the problem, but only part. It does mean that regenerative braking systems are dubiously useful because any energy savings they provide are counteracted by their own weight. They are currently being developed for performance racing not for the efficiency they might provide, but to recapture the stored energy for a burst of acceleration for passing.
It involves the use of additional motor-generators placed on the wheels to recover and deliver energy. Your first bottleneck is the efficiency of those motors. The system can be no more efficient than they are. You then have to deliver that energy to a DC storage system, require voltage regulation and protection. It is also likely to require conversion from an AC waveform. These things consume energy. To actually use that energy, you have to send it through a circuit that converts it back into a waveform that will run the motor. This is not as simple as sending it backwards through the original circuit, although elements can certainly perform double duty. Additional requirements involve a system to monitor the position of the rotor so that the motor doesn't bind or slip. That also consumes energy.
It seems like a great idea to recover heat lost due to friction in the brakes. It looks like tons of free energy. If you use 1/2 m*v^2 to calculate the kinetic energy of a car traveling at city driving speeds, the number is large. The reality of the situation is the number that finds it's way back to the wheels when the light turns green isn't nearly so large.
Then, once you've reused that energy, you must contend with the additional weight that your regenerative braking system adds, and the negative effects it has on the efficiency of the vehicle under normal driving conditions. You also have to factor in the increased cost of design and manufacture.
In a purely electric vehicle, regenerative braking systems are more appealing because they can be tied in to the normal drive and energy storage systems of the vehicle. This reduces energy losses considerably, and greatly simplifies design. Batteries are terrible, their recharge time is slow. To fully implement them requires the capacity to drive them around while they are plugged in to the grid the way some light passenger rail works. A blending of the following projects would be one possible means to achieve that result:
http://www.solarroadways.com/intro.shtmlhttp://www.witricity.com/Finally, as stated earlier, no Amanar, hybrid vehicles are not entirely more fuel efficient than their traditional counterparts. They are more efficient than some of them, less efficient than others. The list fluctuates annually depending on which new year models have been reworked for efficiency.