Had always thought about this but getting a fish so now putting out for comment. Getting a Potter's Angel and thinking the mainly macro ref would be a better place to stay rather than a bare 15g with PVC fittings
Have a 20g ref separate from my sump. Can easily be isolated from rest of the system by existing valves. Worst case if ich or something else develops I move the fish to a bare dedicated QT, and let the 20g ref go fish fallow.
About the only negative is I wouldn't have a ref in-line, but that's if can be worked around by putting the macro in the sump (how I was running before I added the separate refugium).
Old picture, 20g to the right
