He's mostly right. Extra batteries do help with more than just that though..they'll also help regulate voltage while the alternator is at idle (i.e. they help with city driving when theres a lot of stops, etc) and having a battery in the back near your amps, can also have a similar effect as a capacitor.
Also, (this part I can not attest to 100%, because personally I have never tested it, or prooved it by other means to be true, but it's commonly said by others - to take it with a grain of salt) some people claim that like adding a stinger battery is the example I can think of, actually raised their dB scores on a termlab by a noticeable margin over optima's, and the reasoning behind this, is because the battery is able to discharge more power, faster.. So adding a second battery in parallel, would also double the amount of power you were able to draw on (not over time, but like an amp/second ratio). But it would also help over time, too, like Brandon mentioned.
But Brandom was 100% absolutely correct no questions asked - in that there is NO replacement for a HO alternator if you want a good setup. You could add a bunch of batteries, and connect them to a charger every night after playing your system, but this isnt the best route by far - as at a lower voltage (which it will still cause not being able to charge them with a normal alternator as fast as you're drawing power) your amplifiers have to work harder to produce the same amount of output (and actually the lower your voltage drops, the more amps the amplifiers will pull - if they're regulated). If they're unregulated, your amps will be producing a significantly less amount of wattage.