Last I heard, damage reduction reduce a % portion of damage based on some formula capped at some value.

According to test numbers seen, it's less effective than def/res, but more effective than half of them, ergo 2 reduction would be better than 1 def + 1 res (ofc, you can't have 2, but this means it have a higher per-slot value than def/res)

So yes, I do use them. In generality, they are better than defense and is a toss up with resistance, with the latter being better if you have any ability that utilize it (like shield) while damage reduction being better otherwise.

That said, due to the cap, which I do not know what it is, there might be more significant diminishing return on it. Thou I doubt anyone have reached it yet.

Also, as of last confirmed test, *ion does nothing. And to anyone who claims that they have tested and that nin *ion does help with nin damage against resistance or damage reduction, I know that's BS. Because that's not even what it's supposed to do. Granted, it could, because there are intersections where it would actually work that way. But that's not how it works and therefore shows a flawed understanding and, in all likelihood, test design.

I will continue to ignore anyone who claim they work until they show me numbers and testing methodology.