[patrickt]

If you are paying attention to your gauges, have an inquisitive mind and notice small changes and follow up on them, then an ammeter is fine especially if you have a shunt wire. Unfortunately, 99.99% of drivers do not fall into this category so harmless voltmeters and big red flashing idiot lights are needed. A few years ago a slightly wavering ammeter needle helped me spot a deteriorating ground connection on the voltage regulator. An idiot light would never have done that and I don't think a voltmeter would have done so either.