Equipment can fail. Debris can clog media, pumps can randomly stop running, etc. Any of these can completely undo everything in your reactor and result in a crash. Any time you rely on any piece of equipment you are risking a failure.
Live rock removes nitrate. If you have adequate live rock with proper flow you don't need anything else. I see VERY few people using DSBs, but most will use a fuge. There are other benefits of refugiums, and if you have room in your sump you might as well use one, it definitely won't hurt in any way.
My tank had enough rock and proper flow that the rock removed so much nitrate that the chaeto in the fuge couldn't grow.
If someone has been running a nitrate removing reactor for years as the main method of denitrification and can speak for its effectiveness I would love to hear from them.
The definition of a refugium is a safe place, something that the display tank full of fish and other livestock that are happy to eat all those little pods
is not. This is exactly why a refugium is physically separate from the display.