There are a couple of reasons that a refugium might be a good idea even if you have macro in the display.
You can light the macro on a reverse schedule from the main tank, which will help to prevent pH swings at night when macros stop photosynthesizing. Also a refugium can be a place to house brood stock of critters (such as pods, mysis, shrimp, etc.) away from the hungry snouts of the seahorses. This way no matter how good a job the seahorse do of cleaning out these critters in the main tank, you’ll still have a breeding population in the fuge.