Considering the DRK story quests completely drop that angle after lv50, it's reasonable to assume that establishing dominance over the shadow once is enough to keep it in check.
Edit: To elaborate a little bit more on that, as you're growing in power the Darkside entity is also growing in power. You could draw from that that once you hit a certain threshold you're either controlling it or its controlling you. If you can't handle or manage those powers, you succumb to the bad parts. If you can, you tap into the advanced powers.
Edit: To elaborate a little bit more on that, as you're growing in power the Darkside entity is also growing in power. You could draw from that that once you hit a certain threshold you're either controlling it or its controlling you. If you can't handle or manage those powers, you succumb to the bad parts. If you can, you tap into the advanced powers.