It would be fantastic if we had perhaps a toggle-able option to actually see the spawn prevention radius of our cairns. Very often just the one cairn doesn't cover enough area to prevent wandering critters from getting close enough to attack and we wind up building a ring of them, it would be good to see the radius so we're not packing too many of them too close together (and wasting potential construction limit in the process)
It would also help to spot bugs where mobs spawn inside areas that should be protected.