So, even though some of my opinions and methods could no doubt be challenged, she has almost reached the point where she could use manual exposure settings if she so wished. But why? - Except in our macro work, I can't imagine any circumstances where it would be appropriate.
How about pretty much any time when you have constant light upon your subject/scene but a background (or subjects) of ever changing tonal mix?
e.g. when shooting birds in flight and finding the background varying from clear sky to woodland or water.
e.g. when shooting in a studio with off camera flash.
e.g. when taking group shots at a wedding in open shade (steady light) and having an assortment of compositions from the groom and groomsmen in black for one shot and the bride and bridesmaids in pale tones for the next, and every other combination of tones imaginable as everyone else joins in.
e.g. With a subject situated within a snow scene, how much compensation should you dial in for auto modes? What if the subject is wearing black and nearly filling the frame? What if they are part of a long line of skiers playing "trains" with 90% of the scene filled with snow?
I remember a trip to Egypt when I first bought a DSLR and achieving sub-optimal results by relying on auto modes to deal with the lighting. If I had understood things like "Sunny 16" back then and been less afraid of manual mode I'm sure that my results could have been better.
I also remember shooting my first wedding just three months later, as a guest, but nominated "chief photographer" in the absence of a pro. I'd been researching furiously and most advice, especially when using flash, was to shoot with manual exposure for ambient light. That was my intention right up until the first picture, when I bottled it and chose Av instead. Big mistake! Shutter speeds all over the map, and often too slow. Backlight from windows throwing things off. Most indoor shots underexposed. Some outdoor shots overexposed. It was a painful lesson. I don't shoot like that any more.
I've posted this set before, but here is an example of the control and consistency you can obtain by choosing to shoot with manual exposure. These were all shot over a 40 minute period and are unedited. It makes no odds whereabout the subject was within the frame, nor whether the background was mostly snow, mostly sky or a bit of each. The snow stayed white throughout, never grey, never blown out. The exposure was set for the lighting, and that didn't change at all.
(Actually the exposure was set by metering from the sunlit snow and setting an exposure manually to put the brightest parts on the threshold of clipping. That's a technique I commonly use for all sorts of things. Nonetheless, steady lighting made manual exposure a no-brainer, whereas anything else would, IMHO, have been daft.)
In other words it is sometimes far easier to set an exposure for the incident light than to be metering from your subjects/scene and faffing around to compensate for their tonal mix. Sometimes I make mistakes with my manual exposures, but that's usually because I am being lazy, or occasionally can't respond quickly enough to changing light levels. But those occurrences are rare, and in my experience I'd suffer far more pain if I was forever to use one of the auto exposure modes. Of course I will use them if it makes more sense to do so, but for my shooting (when I pay deliberate attention to choosing the lighting conditions) I am better served with manual more often than not.
The simple truth is that there is a time and place for auto and a time and place for manual. Anyone who dismisses either approach out of hand probably needs to review their reasoning.
