Nin and tai *ion overcome the excess of def/resistance granted by damage reduction. If your enemy owns 0 damage reduction secondary stats they are useless, if he owns an high damage reduction stat they are useful.

Just to be more precise: lets say you own 10000 resistance, 9000 defense and 2000 damage reduction. If your enemy hits you with 0 *ions it will hit like if you owned 12000 resistance and 11000 defense. If he hits you owning 1500 tai pen and 1200 nin pen he will hit you like if you had 10800 resistance and 9500 defense, if he hit you owning 3000 tai pen and 4000 nin pen he will not hit you like if you had 8000 def and resistance, but like if you had your basic 10000 res and 9000 def, so those 1000 tai and 2000 nin *ions in excess he stacked were useless in that case.

About elemental resistances i am pretty sure +2 are worse than resistance/damage reduction granted by essences. +3 comparable + 4 better.

Nope, that's not what they are SUPPOSED to do at all....

so obviously you are pulling this out of rumor mill...

Here is a hint, those two stat have NOTHING to do with ANY stat at all. The work in a completely different way than every other stat.

well, supposed to anyway.... there are evidence for them not working at all and no counter-evidence thus far. Even if the evidence tend to be somewhat dated. But until someone actually prove otherwise, might as stay away. Even those who say they work say their effect are minor so....