At macro distances, the movement is primarily forward and back - IS doesn't help in that case. Faster shutter speed [even with higher ISO] or flash is a better answer.
Really? At macro distances and hand-held, if I turn off IS and look throught the VF, I see all kinds of shifting including horizontala and rotational, not just back and forth. IS certainly is not AS useful at macro distances but is still useful for me for hand-held macro shots, making the difference between getting the shot or not on my 100L..