Actually they don't even "fix" the light dimming issue. As the cap discharges, the voltage gradually (relatively speaking, still less than a second) drops down to the level of the battery. The lights still dim but since the dimming is not as sudden you don't notice it as much. A big alternator and a good sized cap can provide stable voltage to a system. A big cap with an insufficient alternator will make a bad situation worse. A big alternator with no cap will see tiny dips in the voltage with bass hits, but not enough to make the lights dim noticably. Cap or not the results are inaudible. Even the most trained ear cannot distinguish 1dB of difference in volume and the difference between a cap and no cap is less than .1 dB. People that claim that they can hear the difference are the same people that "hear" the difference in brands of OFC 12 ga wire and "hear" the difference between Class A and A/B amps. To each his own. In a double blind test they couldn't tell one from the other. They might guess right 50% of the time but a monkey can guess right 50% of the time.
Who are the big proponents of caps? The companies that make and sell them. Who are the proponents of really expensive cable? Mostly the companies that sell them. Who buys them? Suckers that believe the hype. Of course they can "hear" a difference. Psychologically they have to justify the stupid amount of money they just spent on wire and the ears are controlled by the brain. You can convince yourself that you hear most anything, whether your ears actually perceive it or not, if your brain thinks it's there then it is. The human memory of sounds is very short term, generally only a few minutes. We are visual not aural creatures. We are thinking predators, not high strung prey. Sometimes we think too much and our brains over-ride our actual senses.
