You're right that a failure in the D+ circuit would not show up on an idiot light, although I have never seen that problem on any of my cars, my customers' cars, or any other car I've driven. I'm not trying to be confrontational, just saying that an ammeter is of very limited usefulness at best.

I've seen plenty of ammeters in British cars (of a certain age, at least), some of which didn't even have fuel gauges. I don't recall seeing one in any Italian or German car. Volvo didn't see fit to have them, even in the 1800 series, which are otherwise fully instrumented (oil temp as well as oil pressure). I don't suppose Ferrari omitted ammeters because they were trying to be cheap.

I remember a voltmeter in an early-'70s Firebird Formula, maybe just because it was cool to have lots of gauges, and that was a cheap one to add.

OTOH, I've seen lots of charging problems that the light warns of. Total failure (blown rectifier diode, for instance)is a bright light, of course. Worn brushes give a dim light. DF failure gives a light that brightens as revs increase.

I actually had one car come in with a charging problem that turned out to be due to loose connections at the added ammeter. And of course the gauge itself could fail internally and cause lack of charging without telling you anything about it.

It's fine with me if anyone wants to install an ammeter, although I think a volt meter is potentially more useful (as in the case of a shorted battery cell -- you'd see that right away). I'm just saying that for me, personally, either one is w-a-y down the list of instruments I'd want to have.