Flash stops subject motion but I still lose macro shots to camera motion which is worse with a longer lens and greater magnification. This is no problem when the light is dim enough to shoot several stops underexposed at 1/250 but some macro subjects bask in the sun and the flash is more of a fill than the only light to consider.
Example (note motion making crescents out of the specular highlights):
With macro, don't be afraid to try FP (high speed) flash when shooting close and sunny. 1/500 with flash: