Simply peeping at the histogram is too simplistic an approach, one which is not necessarily always appropriate!
Simply spot metering skin is too simplistic an approach...different races have different fundamental skin brightnesses for that approach to work. Even Caucasion skin tans duing the summertime due to sun exposure, lightens in the winter due to lack of sun exposure...that is not a consistent way to determine exposure. And if I spotmeter MrObama's skin in one photo and meter MsClinton's skin in another, they will appear to have identical skin brightness in the two photos, even though we know differently! And the rest of the scene will be overexposed after I spotmeter MrObama's skin, while the rest of the scene will be underexposed if I meter MsClinton's very light skin during the winter.
Some scenes are brighter than average (bride in white against a snow scene), and some are darker than average (black cat in a coal mine)...so simply using the reflective light meter in our cameras can fool the camera into a wrong exposure. Or we can employ EC to help the meter in setting things under Av (or any other mode)
Av is merely one example of Control Setting Automation, and is equally prone to exposure errors made by the meter as any other mode (Green Box, P, Tv, Manual) from full automation to no-automation.
Understand better the principles behind determining 'proper exposure' ...