The main reason I can see for shocking a SWG pool is if you need to shock the pool itself such as if you see algae growing on the walls or something. The water is seeing high enough chlorine levels as it passes through the cell to break down any chloramines in the water, but the pool shell itself never sees Cl levels above the residual you maintain. Textured surfaces such as pebble are probably harder to see the small algae spots if they start, and harder to kill all the algae in the nooks and crannies between the pebbles. I personally use bleach to shock. Di-chlor is not as fast-acting and it adds CYA