i have a refugium in my sump. i think that anytime you can have one you should. anytime you can add more water and flow to you display tank you will benifit the inhabitants of that display tank. if gives a place for certain algeas and pods to breed and live while supplying the display with enough of them to feed certain fish and corals. but this is just my opinion and im sure you will get some replies that are the exact opposite of mine. so it is really up to you, but i suggest one whenever it is possible.