In a reef tank, it is best to not make your sump a refugium. 'Pods
and such will often be destroyed in the trip to the main tank via the main pump. Small snails, bristle worms and such would almost definitely be destroyed. It is best to have a refugium that spills into the main tank.
In a FW
tank, I'm not exactly sure what you would use a refugium for, other than plants. I can see that if you had fish that would eat or destroy plants, you could keep plants in the refugium and still receive the benefits. In this case, you could definitely create a refugium in your sump.