The cap just makes your alternator work even harder. The people who were telling you that are dumbasses.
a cap stores energy
when the energy stored is released the cap must then collect more energy, further draining the electrical system
also:
the amount of charge a cap holds will discharge instantaneously, thus only helping for a fraction of a second. Most amplifiers have a capacitor bank built in to them, performing this same function. The REAL function would be not to hold charge for your amps but to ask as almost a filter for the electrical, stabilizing the voltage long term by aiding the aliternator and battery with it's quick reliease capiblities. However, if your electrical system is up to snuff, you shouldn't have a NEED for this anyway.