I had a dimming problem BEFORE I installed my sound system. After doing the Big3, it's gone. Most cars do gain some electrical performance from a Big3 upgrade, especially if you plan on taxing the electrical system with a powerfull sound system.I'm only running about 800 watts total, but I knew my stock system couldn't handle it. It was already 'iffy. Big 3 and new batteries made big difference.
It's very difficult to calculate your reserve capacity. Once your seeing voltage dips and serious dimming, you are already into a danger area.
Not everyone needs to upgrade their charging system, but most do. Most cars don't have much reserve capacity, and most people want to use large sound systems that go way beyond what the car is designed to handle.
Given how many people we know who are left with dead cars due to electrical failure after installing their sound systems, a Big3 upgrade is cheap insurance and usually does give improved performance and reliablity.
The battery craze right now is a little overblown as well. To many people it's the new "capacitor". I have 2 batteries and an isolator because I need to use my system with the engine off for extended periods of time. Not for bumping in parking lots with the baggy pants crowd.