The point of dynamesh is to rapidly remesh and redistribute polygons as they become stretched or distorted. Distorted polygons are what cause those rough sections…you can only stretch a polygon so far out of its original position before it becomes distorted. In Zbrush terms, an ideal polygon is as close to square-shaped as possible.
Dynameshing frequently redistributes the polygons before they get overly stretched. Sculptris Pro mode makes a great partner to dynamesh though, because it actually creates additional polygons as you sculpt in real time, reducing the frequency with which you might want to dynamesh otherwise.