An ammeter would have actually been useful to me this summer, when the brushes on my rectfier/volt reg wore past the servicible limit, resulting in intermittant alternator ops. (I was able to diagnose a problem with the brush/VR assembly via other means, but an ammeter would’ve made it much easier).
An ammeter would also be useful when one has one’s car jumped: you’d know just how far you’d need to drive to be able to shut down w/ enough juice to start again.