One of the main advantages of an ameter is if you have alot of draw, you can monitor weather or not there is a surplus going to the battery.
However with a volt meter there is no way to tell if you are over drawing the system.
Andrew
If you're drawing the battery down, you'll get a voltage drop, if the battery is charging fully, you'll find your voltmeter reading between 14.2 and 14.6v
This is as a result of Ohm's Law. Voltage=Current X Resistance or alternatively Power (watts)=Current X Voltage.
Amp meters were gone on all cars starting in 1990 and on almost every non mopar vehicle by the mid 70's. If it was better they would not have dropped it.