Spot metering also doesn't work on a dark subject because the camera meter will overexpose to bring the dark subject up to a midtone. If you are working in a studio setting with controlled lighting then the exposure should not change (significantly) with the subject. This is the ideal situation for those of you taking product shots for eBay. If the lighting is changing (e.g. ambient through a window, flash with TTL metering) then it gets more tricky.