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My preference is candid portrait so I'm either on "spot" or "partial" most of the time. I'm usually outside where circumstances can change radically from one shot to the next so I want control of exposure in the face area, which is where I focus and is what I want to show/see. Spot can sometimes be limiting for only the face area, so partial is generally my preference as it meters a slightly wider area that may account for contrast between hair, clothing, hats, etc.
As one person also mentioned, spot is good for macro when you don't want the background affecting the metering of the tiny thing you want to see.
Another situation is night shooting, especially when shooting lights. This meters only the bright light so the light comes out correctly as well as providing a faster shutter speed. This can be good for holiday lighting too.
As for the "skin tone" you mentioned, I don't see exposure affecting this much. And it's easily altered in post if you shoot RAW. More often, in my experience, skin tone is a matter of white balance. This is especially true with women wearing makeup. I've had difficulty with pictures of couples where you can adjust to make the man look good, but the women looks awful. If you make the woman look good, the man looks bad. That takes more than WB and exposure to get a good look.
