Too many are fixed on the idea that mitigation is exclusively damage reduction.

Let's be completely real here, this is all about NF's purpose, and to that end people have worked out the math that it's outright superior to RI so long as the incoming damage isn't lethal. That said, it should be noted that NF isn't a forced choice between healing or damage reduction, you can have both from traditional CDs in combination with NF, and why not use them in tandem? It's free real estate.