Quote Originally Posted by Richard Lane
If the object that you are focusing on is far away, and very small (like the birds head above) and moving slightly, and the background is large and diffuse,and the the lens is not held perfectly still, or is not on a monopod or tripod, then the spot AF and photographer may have a hard time focusing on the subject and the spot AF will have an easier time focusing on a large diffuse background.

This is a common scenario for me - bird surrounded by branches, with the 100-400mm lens at 400mm handheld on my 7D. My settings are Spot AF and spot metering, and I prefer to use an off-center AF point for composition reasons. Here's where back-button AF really helps. The sequence of events goes: center subject in frame, half-press shutter to spot meter (also starts the lens' IS going) and press AE Lock, recompose with selected AF point over subject, press AF-ON to focus, press shutter. All of these were shot using Spot AF: