After reading "This lens is soft" and other myths by Roger Cicala, I decided to bone up on the micro adjustment feature for my Canon 5D Mark II. Essentially, this is designed to correct minor focusing problems. You may have heard the terms "front focusing" or "back focusing." It means that the lens and the camera are not in perfect focusing alignment. According to Cicala, many pro bodies, including the Canon 7D, 1DIII, 1DsIII, 5DII, 50D; Nikon D3, D3x, D300, D700; the Pentax K20D, the Olympus E-30 and E-620 have a microcalibration feature that allows you to better match problem lenses to the camera bodies.
The owner's manual for my Canon 5D Mark II cautions, however, that using micro adjustment should be the exception, not the rule.
"Normally, this adjustment is not required. Do this adjustment only if necessary."
That being said, if you have a camera that provides micro adjustment, and you suspect that a particular lens could use a little correction, you might want to investigate this further.