What he means is that it doesn't make sense to add another car battery just for a project.
When you say wouldn't have to replace it as often, I assume you mean one of two things:
A: The car's battery would be drained by the electronics.
B: The battery will be worn out more quickly by the extra load.
The battery in a car gets recharged constantly as the car is being ran. So it wouldn't be drained by the project. Car batteries aren't replaced because they run low like some AA's in a gameboy. They are replaced because, like any rechargeable, they stop holding a charge after a length of time, or charge cycles. Really, the only time a battery is being discharged in when your car is off, or being cranked. After that, the alternator supplies enough power for everything. You can crank a car, then remove the battery entirely, and it will continue to run! (assuming your alternator is healthy)
As for B, the amount of current that these projects pull is relatively low. Simply turning on your radio easily pulls more than my whole project. Turning on your headlights, even more so.
So adding another battery to power these projects wouldn't make sense. The car's battery keeps itself charged, is already there, and isn't adversely affected by it. (would barely even notice)
Also, from your post here:
http://www.arduino.cc/cgi-bin/yabb2/YaBB.pl?num=1239996764/13#13
You shouldn't really control voltage with just a resistor. The reason is that your voltage with fluctuate depending on how much current your load draws at the moment. Take a look at Ohm's Law, it will explain it fairly well. This is why voltage regulators (such as the 7805) are used. They hold output voltage steady independent of load.