batteries won't stop your voltage from dropping. adding batteries will increase voltage drop (down to the battery float voltage) since another battery is another load for the alt to charge. voltage drop itself is inherent in EVERY wire that has current. can't avoid it.
you only get 14.4V out of an alternator when it can supply enough current to all of the loads in the car (system, battery charging, car stuff, etc.). once you ask for more than the alternator is providing at that instant, voltage drops. the alt output depends on the engine speed and ability to continue to turn the alt pulley when on a load. the alt is a generator, as you load it up it becomes harder to turn. in general, you get about half the alt output at idle compared to 2500 RPM.
so at idle, with your system cranked and headlights on, you'll get dimming regardless of how many batteries you have. batteries can't help you maintain 14.4V. they can help you maintain 12.5-13V. The drop from 14V to 13V is visible in the headlights.
add a volt meter in the dash somewhere (the cig lighter kind is easy). keep an eye on voltage. note that adding batteries means you have even more to charge. and the weakest battery will drain the new stronger battery, pulling it down.
one good starting battery is enough for most systems. adding batteries adds complexity and cost in wiring and fusing, complexity in mounting/safety, and they take up space.
you should have 1/0 wire between the batteries and fuses on each end of the power wire. upgrading the Big 3 is also recommended.
if it were me, i would get the best up front battery you can afford, and do the big 3. then see where you are at. if you only listen when the car is running, and you have a modest system, that should be plenty.
people that add multiple batteries for "daily drivers" may just have inadequate charging systems and rely on the batteries to power the system and supply instantaneous current. additional batteries are certainly good for peak bass demands (at a lower voltage than the alt). so you get voltage drop which reduces power output, but maybe not as much if you didn't have the extra batteries.
that should confuse you more. cheers.