I'm particularly interested in how the author says it's "useless".
This is something that most everyone does but in most cases, it's useless and sometimes dangerous. The '3' are the wires/cable/straps that make various connections. One is from the battery to the body of the vehicle. Another is the wire between the battery and the alternator. The third is the ground between the engine block and the firewall. The connection between the battery and the body and the connection between the engine block and the firewall can be done without any problem. The one from the alternator to the positive battery terminal can cause some safety issues. In some vehicles, there are over-current protection devices in the battery-alternator line. In older systems, the device was a fusible link. A fusible link is a short length of wire that's smaller than the wire it protects. When too much current flows, the wire burns open. For most wire, this would be dangerous but fusible links have a fire-resistant silicone insulation that contains the molten copper. In other vehicles, the protection device is a fuse in the fuse box under the hood. If these are bypassed and there is an alternator failure that causes a direct short to ground, there could be an electrical fire.
Given this information, what exactly is the point of having extra batteries?
Extra batteries are great if you want to listen to your system with the engine off. While the alternator is charging, the extra batteries will only draw current which could otherwise be going to your amplifiers. For proof, all you have to do is measure the voltage while the engine is running. It should be approximately 13.5-14.4 volts DC. Then turn the engine off and measure the battery voltage again. Now it'll be around 12-12.5 volts. Whenever the voltage at the battery is near 14 volts, there is current flowing into the battery. The only time that the battery supplies current with the engine running is when the current capacity of the alternator has been exceeded. When that happens and the batteries are the source of power, the voltage supplied by the batteries will be significantly less than the voltage of an alternator that's capable of supplying the required current.