Auto modes vs Manual mode and choices of metering modes are interesting discussions. I used to shoot slide film, digital has made life easier as you get instant feedback, be it the monitor or the histogram. Personally, I find the histogram more reliable than the monitor.
This set of photos is from a wedding last month. I don’t know if it will clarify anything or just muddy the discussion further, but here are four photos, three viewed in Camera Raw so you can see the histograms and the fourth is the one that was used on my web page.
The wedding was shot by a professional photographer, and this was taken from their web page: Manual mode, 1/160th, f/2.5, ISO 6400, Canon 85 mm f/1.2 L lens
This is one of mine, opened in ACR, with defaults: Program Mode, 1/60th, f/3.5, Exp Comp +1/3, ISO 1600, flash comp 0, Canon 24 mm f/1.4 L lens
This is another of mine, opened in ACR, with defaults: Program Mode, 1/200th, f/1.8, Exp Comp +1/3, ISO 1600, flash comp -2, Canon 24 mm f/1.4 L lens
This is the photo that went on my page: Program Mode, processed in ACR, it was taken between the Program Mode and Aperture Priority photos shown above, with the same settings as the other Program Mode shot, then processed using both Fill and Recovery sliders as well as Lens Correction to remove keystoning.
A final thought for this post is that had I realized this discussion was coming, I would have taken a shot of the cake in manual mode as well, but I don’t bother with manual mode for shots that don’t benefit from using manual mode, so I borrowed someone else’s photo since the subject, time lighting and even camera bodies were similar, they chose a different lens but we both used Canon 5D Mk III bodies. The mode does not make a huge difference to the image unless you want to deviate from what program mode provides, mostly control of aperture to affect depth of field. As far as exposure, you can alter ISO, shutter and aperture; as long as you keep balance the exposure should be the same, and from the examples above we can see that manual mode is not a magic solution.